advanced material solutions for pem fuel cells (phase 2 ... · cells (phase 2) – final report...

Advanced Material Solutions for PEM Fuel Cells (Phase 2) – Final Report

Authors: Elina Yli-Rantala, Pauli Koski, Mikko Kotisaari, Sonja Auvinen, Marjaana Karhu, Juha Nikkola, Pertti Kauranen, Arja Puolakka, Pirjo Heikkilä

Confidentiality: Public

RESEARCH REPORT VTT-R-03694-12

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Report’s title

Advanced Material Solutions for PEM Fuel Cells (Phase 2) – Final Report Customer, contact person, address Order reference Ahlstrom Glassfibre Oy Hydrocell Oy

Project name Project number/Short name Advanced Material Solutions for PEM Fuel Cells (Phase 2)

Kehittyneet Materiaaliratkaisut PEM-polttokennoissa (Vaihe 2) 72419 MARAPOKE2

Author(s) Pages

E. Yli-Rantala, P. Koski, M. Kotisaari, S. Auvinen, J. Nikkola, M. Karhu, P. Kauranen, A. Puolakka, P. Heikkilä

41/

Keywords Report identification code Carbon paper, Gas diffusion layer, Multisinglecell, Proton exchange membrane fuel cell

VTT-R-03694-12

Summary Gas diffusion layers (GDLs) for polymer exchange membrane fuel cells (PEMFCs) were studied with special attention given to the phenomena related to the ageing of the GDLs. Various ex-situ characterization methods for both carbon paper and carbon cloth type GDLs were evaluated. In addition, in-situ characterization of commercial carbon papers in multisinglecell (MSC) setup was developed resulting in one long-term test of about 1200 hours and a great deal of hands-on experience. A technology follow-up related to PEMFCs and GDLs resulted in a technical report of GDL characterization methods, which later on was used as groundwork for a journal article written in cooperation with Arizona State University (ASU).

Confidentiality Public Tampere 16.5.2012 Edited by Elina Yli-Rantala

Research Scientist

Reviewed by Pertti Kauranen

Chief Research Scientist

Accepted by Erja Turunen

Vice President, Applied Materials

VTT’s contact address Dr. Pertti Kauranen, P.O.Box 1300, 33101 Tampere, [email protected] Distribution (customer and VTT) Public

The use of the name of the VTT Technical Research Centre of Finland (VTT) in advertising or publication in part of this report is only permissible with written authorisation from the VTT Technical Research Centre of Finland.


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Preface

The work described in this report has been carried out by VTT in Advanced Materials and Fuel Conversion Knowledge Centres, and by Tampere University of Technology in Fibre Material Science Group (TUT FMS), in the time frame of 1 August 2010 to 31 March 2012. The work has been funded by Finnish Funding Agency for Technology and Innovation TEKES under the contract number 40126/10.

The steering committee consisted of the following representatives: VTT Dr. Erja Turunen TUT FMS Dr. Pirjo Heikkilä/Ms. Arja Puhakka Ahlstrom Glassfibre Oy Ms. Hanna Rahiala Hydrocell Oy Mr. Tomi Anttila Tekes Dr. Markku Lämsä The funding by Tekes and the industrial companies, as well as the diligent cooperation between all the parties are gratefully acknowledged. Tampere/Espoo 16.5.2012 Authors


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Contents

Preface ........................................................................................................................ 2

Commonly used abbreviations .................................................................................... 4

1 Introduction ............................................................................................................. 5

2 Goal ........................................................................................................................ 5

3 Description.............................................................................................................. 6

3.1 GDL in PEMFC ............................................................................................... 6 3.1.1 Functions and structure of the GDL ..................................................... 6 3.1.2 Degradation of the GDL ....................................................................... 7

3.2 Characterization approaches .......................................................................... 7 3.3 Studied materials ............................................................................................ 7 3.4 Technology follow-up ...................................................................................... 8

4 Methods .................................................................................................................. 8

4.1 In-situ characterization methods ..................................................................... 8 4.1.1 Multisinglecell setup ............................................................................. 8 4.1.2 Multisinglecell operation ..................................................................... 10

4.2 Ex-situ characterization methods .................................................................. 11 4.2.1 Mechanical properties ........................................................................ 11 4.2.2 Porosity .............................................................................................. 14 4.2.3 Permeability ....................................................................................... 14 4.2.4 PTFE content ..................................................................................... 15 4.2.5 Electric conductivity ........................................................................... 15 4.2.6 Thermal conductivity .......................................................................... 17 4.2.7 Structure ............................................................................................ 17

5 Results and their validation................................................................................... 17

5.1 Multisinglecell ................................................................................................ 17 5.2 Ex-situ characterization ................................................................................. 25

5.2.1 Mechanical properties ........................................................................ 25 5.2.2 Porosity .............................................................................................. 28 5.2.3 Permeability ....................................................................................... 29 5.2.4 PTFE content ..................................................................................... 31 5.2.5 Electrical conductivity ......................................................................... 34 5.2.6 Thermal conductivity .......................................................................... 35 5.2.7 Structure ............................................................................................ 35

6 Conclusions .......................................................................................................... 37

7 Summary .............................................................................................................. 39

References ................................................................................................................ 40


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Commonly used abbreviations

AFM Atomic force microscopy ASU Arizona State University BET BET (Brunauer, Emmet, Teller) surface area measurement EIS Electrochemical impedance spectroscopy EDS Energy dispersive X-ray spectroscopy DMA Dynamic mechanical analysis CCM Catalyst coated membrane GDL Gas diffusion layer HFR High-frequency resistance iAFM Conductive AFM IRD IRD fuel cells (company) KES Kawabata evaluation system MEA Membrane electrode assembly MFC Mass flow controller MIP Mercury intrusion porosimetry MPL Microporous layer MSC Multisinglecell OCV Open-circuit voltage PEMFC Polymer electrolyte fuel cell PTFE Polytetrafluoroethylene SEM Scanning electrode microscopy TGA Thermogravimetric analysis TUT Tampere University of Techology


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1 Introduction

Proton exchange membrane fuel cell (PEMFC) is a promising power source for portable, automotive and stationary applications due to its high power density and environmental benefits [1]. However, durability issues of the PEMFCs still need to be solved in order to make them more appealing for commercialization [2]. International progress in increasing the durability and understanding of the degradation mechanisms of PEMFC components such as electrolyte membranes and noble metal catalysts has been fast. The durability and degradation of gas diffusion layers (GDLs), on the other hand, have received much less attention which could lead to a situation where the carbon paper becomes the weak and durability limiting component in PEMFC. Therefore, more profound understanding of the GDL performance and degradation is needed. [3] A GDL typically consists of a thin layer of carbon black mixed with polytetrafluoroethylene (PTFE) that is coated onto a sheet of macro-porous carbon fiber substrate. It allows the gaseous reactants to move towards the catalyst layer. Due to its electrical and thermal conductivity it also provides a path for electrons and heat to flow between catalyst layers and bipolar plates and plays a critical role in water management within the cell. Although the GDL is a seemingly minor component in a fuel cell, it has been shown that altering the characteristics of the diffusion layer can lead to substantial improvements in the performance of the cell. [3] Different GDL manufacturing technologies were studied in the previous Advanced Materials Solutions for PEM Fuel Cells (Marapoke) project in 2007-2010. The objective of this follow-up project was to gain a deeper understanding of the degradation issues and characterization methods for the GDL materials.

2 Goal

The main objectives of the project have been to 1) develop both ex-situ and in-situ test methods for carbon paper type GDLs in

order to find correlations between the structural, mechanical and electrical properties of the paper and the fuel cell performance,

2) optimize the multisinglecell setup for cost-effective study of GDL degradation phenomena,

3) and as a result of the first two objectives, achieve a fundamental understanding of the carbon paper degradation in PEMFC environment under different operating conditions.

In addition, international progress on GDL development and durability was to be followed. The project activities aimed at helping the participating industries to build up their competence in the characterization and application of carbon papers for fuel cell use. Ageing mechanisms and methodology development results were considered as common interest and the idea was to publish them either in scientific journals or report them in form of technical reports.


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3 Description

3.1 GDL in PEMFC

3.1.1 Functions and structure of the GDL

GDL is a crucial component in the PEMFC stack. The four primary functions of the GDLs are to provide 1) the mass transfer of reactants (fuel and oxidant) and product water between

the flow field channels and catalyst layer, 2) electronic conductivity between the bipolar plates and catalyst layers, 3) heat removal from the membrane electrode assembly (MEA) towards the

coolant channels of the bipolar plates, and 4) mechanical support for the MEA. [4] Figure 3.1 illustrates the location of the GDL in the PEMFC stack. The heart of the stack is a catalyst-coated membrane (CCM) consisting of a proton-conducting ionomer membrane and catalyst layers on its both sides. The GDLs are situated between the CCM and bipolar plates on both cathode and anode sides of the cell. The outermost functional components are the bipolar plates providing the flow field channels for the fuel and oxidant feed and product removal.

Figure 3.1. Schematic of PEMFC stack components.[5]

The GDL consists of a macroporous carbon cloth or carbon paper substrate that is optionally coated with a thin microporous layer (MPL) made of carbon black. Carbon fibers of the substrate are graphitized at high temperature (>2000 °C) to enhance electronic conductivity, corrosion resistance and mechanical strength, and impregnated with thermoset resin to manufacture carbon papers. Carbon cloths are produced by spinning and weaving of carbon yarns, followed by graphitization. The GDL is typically wet-proofed with polytetrafluoroethylene (PTFE) in order to prevent liquid water from clogging into the pores, which could impede gas transport within the GDL. [6] The first layer in contact with the gas flow channel is the carbon substrate, serving as a gas distributor and a mechanical support for the electrode. The second layer, MPL, enhances the water management, reduces the contact resistance between the


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catalyst layer and the macroporous substrate, and prevents precious catalyst particles from falling inside the substrate. [6]

3.1.2 Degradation of the GDL

As already stated, the degradation of some critical components of the PEMFC, such as membrane and catalyst layers, has received more attention than that of the GDLs. [3] In the past GDL and PEMFC studies, the impact of the GDL material and design on the PEMFC performance has been the focal point instead of the durability issues. However, an increased GDL surface hydrophilicity, which exposes the cell to flooding, has been clearly observed in long-term operation and in freeze/thaw cycling, which indicates that further investigation of the GDL durability is necessary. [7] Unlike the carbon black in the catalyst layer, the carbon black in the MPL is not susceptible to electrochemical corrosion and contains no Pt to catalyze oxidation reactions, but chemical surface oxidation by water or even loss of carbon due to oxidation to CO or CO2 cannot be excluded. Oxidation of the carbon species leads to decreased hydrophobicity of the material. The carbon fibers of the substrate may be more stable, but otherwise are susceptible to the same reactions. Also decomposition of PTFE, which would further decrease hydrophobicity, has been suggested. [2]

3.2 Characterization approaches

The approach to the characterization methods for GDLs was ample. The idea was to analyze the applicability of as many methods as possible. Some of the methods were already recognized as useful tools for the purpose, and others needed further verifying. Another point of view to the characterization was based on mapping the differences that the aging would cause in the GDL, that is, increase in hydrophilicity due to carbon oxidation and PTFE decomposition. All the possibly suitable characterization methods with these aspects in mind, especially those available at VTT or TUT, were evaluated. Mechanical characterization methods included techniques for tensile properties, and dynamic mechanical analysis for compressibility. Porosity was studied by mercury intrusion porosimetry and air permeability by so called Gurley method. Water vapor permeability was in addition determined by a method designed for textile characterization. PTFE content of the GDLs was indirectly investigated by sessile drop method and directly by thermogravimetric analysis. Both electrical and thermal conductivities were studied, and 3D profilometry and atomic force microscopy were used as imaging tools.

3.3 Studied materials

The selection of the commercial materials for the validation of the characterization methods was based on their common use in PEMFCs. SGL Sigracet® 35 series and Sigracet® 34 BC from the 34 series were chosen from the class of non-woven carbon papers. CARBELTM CL Gas Diffusion Media manufactured by W.L. Gore & Associates, Inc. was selected from the woven carbon cloths. Carbel is coated with a microporous layer. Table 3.1 shows the distribution of Sigracet samples into MPL coated and those without an MPL.


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Table 3.1. Classification of the commercial materials used in the project.

Carbon paper (non-woven) Carbon cloth (woven) Without MPL Sigracet 35 AA, 35 BA -

With MPL Sigracet 34 BC, 35 BC, 35 CC, 35 DC Carbel The coding of the Sigracet samples reveals their PTFE content and relative porosity. The two letters denote the amount of PTFE in the substrate and MPL, respectively, as follows:

o A 0 % o B 5 % o C 10 % o D 20 %

As an example, 34 BC contains 5 % PTFE in the substrate and 10 % in the MPL. Series number 34 refers to lower porosity than that of series 35.

3.4 Technology follow-up

Following the technological development in the field of GDLs was one objective in the project. The technology follow-up included taking part in workshops and exploiting other networking actions, as well as composing a literature review with a topic “Characterization of GDLs for PEMFCs”. The literature review was published both in form of a technical report for the benefit of the industry partners in the project, as well as a journal review. The journal article [8], submitted to Journal of Power Sources, was written in cooperation with Dr. Kannan from Arizona State University (ASU). Dr. Kannan spent four months at VTT, from September to December in 2011, and during his stay the project was greatly benefited by his expertise in PEMFCs and GDLs.

4 Methods

The characterization methods were divided in in-situ and ex-situ techniques. In-situ methods comprise multisinglecell testing in fuel cell environment, whereas ex-situ methods covered all other characterization techniques, presented in Chapter 4.2.

4.1 In-situ characterization methods

The in-situ characterization was performed in multisinglecell (MSC) at VTT in Espoo. The idea of the MSC is to combine several single cells into a stack. The cells have equal operating conditions, which makes the data harvest effective.

4.1.1 Multisinglecell setup

In this project the fuel cell test setup consisted of an 8-cell MSC and extensive equipment for operating and monitoring it. The outlet gases were cooled by a cooling cascade, the condensed water collected, and the dry gases led to a 16-channel gas flow meter. Figure 4.1 shows the setup diagram.


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Figure 4.1. Diagram of the MSC setup.

A Greenlight G60 test station provided humidified and heated reactant gases as well as coolant for controlling the fuel cell temperature. The test station was controlled with a LabView-based software ran on a computer incorporated in the system. As for monitoring, the station was used for measuring inlet gas pressures, dew points and temperature as well as cell temperatures and naturally the cell voltages. The controllable variables included hydrogen and air flows including the gas humidity and temperature. The load unit was also controlled within the same station. In addition to these fairly normal capabilities, G60 was also equipped with EIS hardware. The test station mass flow controllers (MFCs) and other equipment were calibrated and tested according to the maintenance schedule provided by the manufacturer.

For measuring the outlet gas flows a flow meter was assembled, see Figure 4.2. The flow meter had two channels for each cell resulting in 8 channels for air flow and 8 channels for H2 flow, with sensors giving a voltage output signal. These voltages were recorded by a computer and manually converted to flow rates in Excel. The outlet gases from each anode and cathode in the fuel cell were first condensed in the cooling cascade and the water was collected in bottles. This was because the gas flow meter required dry gases. The gas flow meter signal was logged by a data logger Agilent 34970 (not in figure) and recorded by an external computer. In addition to gas flow rates, the amount of product water could be measured, resulting in more data. Later on the flow meter box was equipped with new LabView-based monitoring software, which allowed a real-time monitoring of flow rates. This was a major improvement to the earlier software which was only able to display the output voltage levels of the flow sensors.


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Figure 4.2. Image of the in-house built flow meter for outlet gases.

After building up the setup, a lot of troubleshooting and tuning was required. This included fixing of the parameters to fit on the one hand the project purposes but on the other hand the characteristics of the setup. The work involved among other things programming scripts for an automated humidity sweep and polarization run. The high temperature requirements (80+ °C) also required more effective sealing and gas fitting materials. The original Viton seals were change to polyolefin sealing gaskets manufactured by Freudenberg. The cell body gas fittings were also changed to more heat tolerant polyvinylidene fluoride (PVDF) fittings.

4.1.2 Multisinglecell operation

Operation of the MSC included humidity sweeps for accelerating the aging of the GDLs, and polarization runs and impedance spectroscopy for checking the status of the fuel cell performance. General operation parameters were:

o I = 13.2 A (current density i = 600 mA/cm2) o H2 = 3.5 (H2 flow rate 2,57 lpm) o AIR = 1.8 (Air flow rate 3,16 lpm) (changed to 2.0 during tests) o TWATER HEATING = 83 °C o TINLET GAS = 95 °C o TINLET GAS DEW POINT = 78 °C

The humidity sweep was used to fasten the ageing of the fuel cell and particularly the GDLs. The parameters of the humidity sweeps were:

o TINLET GAS DEW POINT sweep from 78 °C up to 83 °C (50 min ramp + 20 min wait), down to 70 °C (130 min ramp + 20 min wait) and finally back to 78 °C (80 min ramp).

o Electrochemical impedance spectroscopy (EIS), see description below, was done before first ramp and at the end of the 20 min steady dew point wait periods

o Change of 1 °C takes 10 min o Duration: 6.33 h o Frequency: once a week


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Polarization runs were used as a general experiment for the fuel cell performance. In polarization runs a constant gas flow rate was used, in contrast to normally used constant stoichiometry. For this reason reactant starvation limited the maximum current density. The run was two-way. First the current density was increased with i = 50 mA/cm2 increment, corresponding to 1.1 A/min, from open circuit to minimum voltage of 0.4 V, after which the current was decreased back to open circuit. The step duration comprised of 60 s of stabilization and 30 s of voltage recording. Polarization scans were recorder at 24, 500 and 1000 hours of operation. Electrochemical impedance spectroscopy (EIS) was performed after the first polarization scans and after this during the humidity sweeps. The measurement was performed for each cell individually, by changing the voltage measurement cables by hand. Measuring the 8 cells took around 15 minutes so it could be done during the steady state plateaus of humidity sweeps. Parameters for the EIS were:

o 600 mA/cm2 with the nominal operating conditions o Range: 5 kHz to 0.5 Hz 10 points per decade o Amplitude: 30 mA/cm2 (5 % of 600 mA/cm2)

Two distinct arcs are visible in each impedance spectrum. The high frequency arc (left) corresponds to the cathode charge transfer resistance and double layer capacitance while the low frequency arc (right) arises from diffusion and mass transfer. Exhaust water collection bottles were emptied at maximum of three day intervals, as otherwise the water collection bottles would flood. To prevent any water from entering the flow sensor system, a humidity sensor was set to monitor the gas water content and perform an emergency shutdown in the case of exhaust gas water dew point raising above 10 °C. Upon bottle emptying, the water amounts were measured for further analysis.

4.2 Ex-situ characterization methods

All the ex-situ measurements were conducted at VTT, Tampere, unless stated otherwise.

4.2.1 Mechanical properties

4.2.1.1 Compressibility

4.2.1.1.1 Dynamic mechanical analysis Compressive behavior was studied by dynamic mechanical analysis (DMA). DMA instrument is designed for dynamical mechanical analysis of materials as a function of temperature, time and frequency. The instrument used in this project, shown in Figure 4.3, was DMA/SDTA 861e by Mettler-Toledo. In DMA measurement a sample is subjected to a sinusoidal mechanical deformation of a certain frequency and the corresponding forces are measured. The measured raw data (measured force and displacement amplitudes and their phase shifts) is used to calculate the desired dynamical mechanical material properties.


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Figure 4.3. DMA measurement device used for determining the compressibility of GDLs.

To evaluate the compressive behavior of the samples, the DMA measurements were performed in compression mode as a function of static force at room temperature. A pure static compression test is not possible with the DMA because the instrument is designed for dynamical testing. Therefore, some dynamic force with a certain frequency is always subjected to the sample. In these measurements the effect of dynamic deformation was minimized by selecting the dynamic force value to 0.01 N and frequency value to 0.1 Hz. The stress-strain curves for the samples were constructed from the measured raw data values for static force and corresponding displacement values. The maximum combined force of the DMA device was 40 N which limited the sample size.

4.2.1.1.2 Kawabata evaluation system So called Kawabata Evaluation System (KES) is widely used in the textile industry for objective measurement of the mechanical properties of textiles [9]. Although the KES has achieved some acceptance in the paper industry as well, it is far from being an industrial standard, partly due to the testing conditions and instruments not being optimized for paper samples. [10] However, the applicability of KES for determining the GDL compressibility was evaluated at TUT. The tester is presented in Figure 4.4 as an image and in schematic form. The measurement practice comprised of compressing the sample with plunger descending at the rate of 1 mm/50 sec up to 50 gf/cm2 (approx. 4.9 kPa) compression pressure and then releasing it. Thickness of the sample was recorded during pressing and recovery. By this method four values were obtained: compressional energy (WC), compressional resilience (RC), linearity of compression thickness curve (LC) and compression rate (EMC). WC is obtained by integration of compression curve, RC is the ratio between recovery and compression energies, LC is obtained by compared pressure curve to linear line and ECM is compression-%.


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Figure 4.4. Kawabata compression measurement unit and its schematic illustration.

4.2.1.2 Tensile properties The tensile properties were determined by universal materials testing machine at TUT. The used device, Testometric M500, which is a constant-rate-of-extension type tensile tester, is shown in Figure 4.5. Test piece width was 20 mm, sample length 100 mm and drawing speed 100 mm/min. Two test pieces per direction (L = long and C = cross) per sample were cut from each sample. The long direction corresponds to the machine direction in the manufacturing process. Stress-strain curves were plotted for both directions of the sample, and the average of Young’s modulus, force at break, strain at break, and modulus at break were calculated.

Figure 4.5. Tensile testing machine Testometric at TUT.

4.2.1.3 Bending Bending strength was measured according to draft of International Standard ISO/DIS 5628 (Paper and board – Determination of bending stiffness – General principles for two-point, three-point and four-point methods). Three-point bending method was chosen for the purpose. The same materials testing machine was used as in determining the tensile properties. Bending length was 50 mm, width of sample 40 mm, deflection 6.6 mm and bending time about 6 s.


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4.2.2 Porosity

4.2.2.1 Gas adsorption method Porosity characteristics were determined at TUT by Coulter Omnisorb 100CX gas adsorption instrument based on nitrogen adsorption and desorption isotherms. Sample, weighing 0.1118 g, was degassed at 400 °C until vacuum reached at least 10-4 Pa and the final weight of the sample was 0.0572 g. Isothermal measurements were done at 77 K using N2 gas of which purity was 99.9999 %. Adsorption and desorption isotherm data were measured with 30 Torr dosage at measurement range 0 < (P/P0) < 0.981 and 0.981 > (P/P0) > 0.3 respectively, where P0 ~ 750 Torr. Surface area of the sample was determined with BET (Brunauer-Emmet-Teller) equation from adsorption isotherm. HK (Horvath-Kawazoe) method was used for the determination of micropore size distribution form adsorption isotherm. Mesopore size distribution was determined with the BJH (Barret-Joyner-Halenda) method from desorption isotherm.

4.2.2.2 Mercury intrusion porosimetry Mercury intrusion porosimetry (MIP) was used for pore characteristics determination. The method is based on measuring the amount of mercury penetrated into pores of the material as a function of the applied pressure [1]. The measurements were conducted at Åbo Akademi, and the results included pore size distribution, total pore volume, total pore surface area and average pore diameter.

4.2.3 Permeability

4.2.3.1 Air permeability

4.2.3.1.1 Gurley method Air permeability measurements were conducted by a Gurley Precision Instrument, which conforms to standard of ISO 5636-5 (Paper and board - Determination of air permeance and air resistance (medium range) - Part 5: Gurley method) commonly used in air permeability tests of various materials. The tests were performed by VTT Expert Services in Espoo in conditions of 50 % RH and 23 °C.

4.2.3.1.2 Karl Schroder tester An air permeability tester D-6940 from Karl Schroder Instruments at TUT was used as a second method for determining the air permeability. This tester is designed to monitor the air permeability of textile fabrics as per the specifications detailed in EN ISO standard 9237 (Determination of permeability of fabrics to air). The rate of flow of air passing perpendicularly through a given area of sample was measured at a given pressure difference across the sample test area over given time period. Used test area was 20 cm² and pressure difference 200 Pa. Results are measured as l/min and given in n mm/s (note that l/m²·s = mm/s). Number of parallel measurements was 10.

4.2.3.2 Water vapor permeability The method for determining the permeability of water through the GDL was based on SFS-EN ISO standard 15496 (Measurement of water vapor permeability of textiles for the purpose of quality control). The setup was constructed at TUT according to the standard. The specimen was placed, together with a waterproof but highly water-vapor-permeable, hydrophobic, microporous membrane (Goretex), on a ring holder and then put in a water bath so that the membrane was


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in contact with the water. This was then left for 15 min. A cup containing saturated potassium acetate solution (approximately 120 g), creating a relative humidity of about 23 % at the specimen’s upper face, and covered with a second piece of the same membrane, was weighed and then inverted above the specimen in the ring holder, so that the membrane was in contact with the specimen. This resulted in a net transfer of water vapor through the specimen from the water side to the cup. After 15 min the cup was taken off and re-weighed. At the same time a control test without a specimen was carried out to determine the water vapor permeability of the two membranes. The water vapor permeability of the specimen could then be calculated, correcting for the influence of the two membranes.

4.2.4 PTFE content

4.2.4.1 Energy dispersive elemental analysis Fluorine content was measured using INCA Energy 350 energy dispersive elemental analyzer (EDS) INCAx-act detector at TUT. An areal measurement of full screen (around 9000 µm2) was used and the sample was assumed to contain only carbon and fluorine, since other elements were not giving signals recognized by the program.

4.2.4.2 Sessile drop method Surface energies were determined by sessile drop method. Surface energy is related to contact angle, which in turn is influenced by the hydrophobic or hydrophilic nature of the surface. As PTFE is highly hydrophobic, this method was used to evaluate the PTFE content of the samples. In sessile drop method, a droplet of liquid is set on the surface and the contact angle is measured by fitting a tangent to the three-phase point where the liquid surface touches the solid surface. Surface energy of the material can be calculated, if at least three different liquids are used. [4] Water, diiodomethane, and formamide were used in these measurements conducted at VTT, Espoo. The OWRK (Owen, Wendt, Rabel, Kaelble) method was used as an analyzing tool. Contact angle measurements were also conducted at TUT by Pocket Goniometer PG-3, which consisted of a drip unit, a video camera and a computer with software for processing the results. The setup determined the contact angle of the droplet as a function of time during 6 seconds. The measurements were only conducted with water, so surface energy values could not be calculated.

4.2.4.3 Thermogravimetric analysis Thermogravimetric analysis (TGA) was performed in order to evaluate the applicability of the method on PTFE content determination. The samples were heated up to 900 °C under nitrogen atmosphere, and the consequent mass losses, related to the decomposition of PTFE, were observed. The measurements were performed at TUT by Perkin Elmer STA 6000, Simultaneous Thermal Analyzer.

4.2.5 Electric conductivity

Electric conductivity of the GDL was examined in both through-plane and in-plane directions.


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4.2.5.1 Through-plane electric conductivity The in-house constructed measurement setup for the determination of through-plane resistance of GDLs is presented in Figure 4.6, along with the electric diagram for 4-point probe configuration. The 4-point probe configuration is to be used when measuring small resistances, in order to avoid the internal resistance of the measuring devices interfering with the measurement. [11] The rods of the measurement jig were connected to both a voltmeter and a power source as schematically shown in the electric diagram on the right-hand side of the figure. An additional current meter was also used to verify the desired current given by the power source. The measurements were conducted with 1 A/cm2 under 1 MPa, which is the commonly used in clamping pressure in PEMFCs [12].

Figure 4.6. Left: Measurement jig for the determination of through-plane resistivity of the GDL samples. Right: Electric diagram of the four-point probe configuration.

Through-plane electric conductivity was also measured at TUT. The measurement setup is shown in Figure 4.7. Measurements were done with current density 1 A/cm2 using 10 kg/cm2 pressure. Surface area of the electrodes was 1.13 cm2, resulting in pressure of about 0.9 MPa.

Figure 4.7. Through-plane conductivity measurement setup used at TUT.


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4.2.5.2 In-plane electric conductivity The in-plane resistivity (or volume resistivity) measurements were conducted by a 4-point probes method with a measurement jig from Parker Chomerics, shown in Figure 4.8. The resistivity values were directly read from an ohm meter.

Figure 4.8. Scheme of the measurement jig for volume resistivity measurements.

4.2.6 Thermal conductivity

The applicability of HotDisk TPS 2500S instrument on measuring the through-plane thermal conductivity of the GDLs was tested. This instrument is designed for analyzing thermal transport properties covering an extensive range of materials of various types of geometry and dimensions. The TPS 2500 S meets ISO Standard 22007-2.2 (Plastics – Determination of thermal conductivity and thermal diffusivity – Part 2: Transient plane heat source (Hot Disk) method).

4.2.7 Structure

4.2.7.1 3D laser profilometry 3D laser profilometry was used as an imaging tool to examine the MPL surface topography and roughness of the GDLs. A 20 x magnification was used, and the imaged area was 637 x 477 m2. The primary profiles, reflecting the surface roughness, were calculated based on area and line calculations.

4.2.7.2 Atomic force microscopy Topography and phase imaging by non-contact tapping mode of atomic force microscope (AFM) was performed in order to examine the microporous structure and composition of the GDLs. The used device was AFM System XE-100 by Park Systems. Topography and phase imaging modes were used, and imaged areas were 1 x 1 m2, 5 x 5 m2, and 10 x 10 m2. In addition, the applicability of conductive AFM imaging (iAFM) run in contact mode was tested.

5 Results and their validation

5.1 Multisinglecell

5.1.1 Assembled MSCs

The naming of the MSC assemblies was kept very generic for simplicity. o MSC0 was the very first test cell, with some shortcomings in assembly. o MSC1 was used for testing, but since the experiment often crashed, the

cell was most probably damaged, although it could still be operated.


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o MSC2 was assembled in hope to start the first scientific test. However, because of breaks of several hours, and because of changing the test parameters along the way, it had a relatively rough history so the data was not considered that comparable.

o MSC3 was assembled as a regular fuel cell, but MSC4 merely for MEA testing. The reason was that the overall fuel cell performance was found to be rather low. The current density was not as high as could be expected, and the problem seemed to be the MEAs. An older batch of Gore MEA was tested in MSC4 for comparison, and the performance was significantly higher. Unfortunately the durability of that particular old MEA batch was too low for the planned long term runs.

o MSC5 was assembled with the low performing Gore MEAs while waiting for the delivery of new IRD MEAs, in order to check that everything was ready for MSC testing with them. The MSC5 had an update in the cooling gasket material. The old fluorinated rubber gaskets were replaced by new durable Freudenberg FC LOS (P78) FC-PO polyolefin gaskets.

o MSC6 was the first assembly with a successful long-term run. The MSC was assembled with 35 BC, 35 CC, 35 DC and 35 BA GDLs. Each GDL material was assembled in two cells forming an 8-cell stack. The stack was run for 1175 h. The life-time events are presented in Table 5.1.

Table 5.1. Life-time events of the long-term run of MSC6 assembly.

Time Event 0 h System startup 0.5 h Gas feeds on 1 h Load at 200 mA/cm

2

1.5 h Break-in cycle attempt and Emergency stop: load fault System back online at 600 mA/cm

2 (0.5 h break)

3 h Load cycled 600 – 700 – 800 – 600 mA/ cm2

Left to precondition overnight 23 h 2 x Polarization scan 25 h 3 x EIS

29 h Emergency stop: hydrogen gas line pressure shock System back online at 600 mA/cm

2 (1.5 h break)

145 h 3 x EIS 168 h Cathode flow from 3.16 to 3.51 nlpm 190 h Humidity sweep + 3 x EIS 311 h Emergency stop: load fault (electricity grid failure) 335 h System back online at 600 mA/cm

2 (24 h break)

360 h Humidity sweep + 3 x EIS

411 h Emergency stop: Cathode pressure over 200 mbar Recovery in 20 minutes

525 h Humidity sweep + 3 x EIS 556 h 3 x Polarization scan 695 h Humidity sweep + 3 x EIS 860 h Humidity sweep + 3 x EIS 1030 h Humidity sweep + 3 x EIS 1081 h 2 x Polarization scan 1175 h Shutdown


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5.1.2 Long-term test

The test was run without any significant problems, excluding the few emergency stops mentioned in the table above. The emergency stop occurred at 311 h turned off the data logging, but it was noticed and turned back on only just one week after the stop due to the holiday season. Therefore no data was recorded during the period from 311 h to 479 h, except for EIS measurements during the humidity sweep. The individual cell voltages recorded during the measurement are shown in Figure 5.1. The figure also illustrates the measurement events, including polarization scans and EIS. All of the tested cells showed very high voltage degradation. During the first half of the test, cells 2-6 had a sufficiently steady voltage degradation rate of 150 V/h to 160 V/h. On the first cell, degradation rate was higher, around 180 V/h. The last two cells (7 & 8) seemed to be degrading much faster, at the rate of 220 V/h and 280 V/h, respectively. Operation at higher current densities during the polarization scans momentarily increased the cell voltages, but the decay continued shortly after. Furthermore, the 24-hour emergency stop with heating at 80 °C also decreased the degradation rate. After 700 hours of operation, the voltage decay started to slow down, except for cell 5.

Figure 5.1. Long-term voltage evolution of each cell of the MSC6 assembly.

The results from the polarization scans are shown in Figure 5.2. All cells showed a significant decrease on the overall performance. The mass transfer losses and ohmic losses on the linear part of the slope did not show any significant changes during the course of the measurements. Instead, the decreasing open-circuit voltage (OCV) and increased activation losses seemed to address for most of the performance losses which are attributed to CCM degradation. The OCV degrease could be even an indication of pinhole formation in the CCM membrane. Also, cells with 35 BA GDL showed higher ohmic resistance compared to other cells.


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The increasing degradation on cell 5 was also clearly visible between 556 h and 1081 h polarization curves.

Figure 5.2 Polarization curves measured during the long-term test.

The results from the EIS measurements are shown in graphs through Figure 5.3 to Figure 5.6. The figures show EIS measured during normal operating conditions, during high humidity part of the sweep and during low humidity part of the sweep.

All of the curves show two distinct arcs, the leftmost one corresponding to reaction kinetics (high frequency) and right one corresponding to mass transfer (low frequency). In addition, the high frequency resistance (HFR) can be estimated by extrapolation as the intersection point of the real axis and the high frequency arc. Comparison between every two cells assembled with the same GDLs showed differences between each pair. For example impedance of cell 1 has a high low frequency arc while on cell 2 it was much smaller. During humid conditions, the difference grew even further as cell 1 seemed to be subjected to some water condensation resulting in a very high low frequency arc. The same was evident between cells 3 and 4. At dry operating conditions, the differences were much smaller. During normal conditions, HFR was between 0.05 cm2 to 0.10 cm2 but cells 7 and 8 with 35 BA GDL had higher HFR, 0.15-0.20 cm2. Depending on the cell, HFR changed during the humidity sweeps. In dry conditions, HF-resistance increased in all the cells indicating membrane drying. In humid conditions, the HFR decrease was more subtle. The effect of 24 h emergency stop at 335 h was also clearly visible on the EIS results. In most cells the light blue curve measured after the stop showed the lowest impedance levels during the whole test. However, excluding the effect of the 24 h stop, the impedance curves stabilized on certain level. At normal conditions and after 500 h of operation, the first six cells produced almost


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identical EIS results with respect to time. Cells 7 and 8 showed more variation after 500 hours and opposed to high frequency 45° angle to real axis, the graphs started to curve upwards at the end. It should also be noted that impedance on the highest frequencies from 5 kHz to 1 kHz were probably distorted due to the load unit frequency limitations above 1 kHz. This is the leftmost part of each EIS graph with a small upwards curl. Also, the abnormal impedance of cell 8 at 190 h (humid conditions) was due to bad voltage reading contacts during the particular measurement.

Figure 5.3. Impedance spectra of Sigracet 35 DC.

Figure 5.4. Impedance spectra of Sigracet 35 CC.


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Figure 5.5. Impedance spectra of Sigracet 35 BC.

Figure 5.6. Impedance spectra of Sigracet 35 BA.

The evolution of the radii of the high and low frequency semicircles did not show any clear time dependence and no direct correlation with the polarization performance. The reversible changes in the low frequency arcs, especially under humid conditions, indicate reversible flooding of the corresponding cells. The flooding was more severe for cells 1 (35 DC) and 4 (35 CC) than for cells 5 and 6 (35 BC) which could indicate that increasing PTFE content can cause problems under high humidity operation. However, more experiments would be needed to verify this. Water accumulation rate from each cell was recorded throughout the test. The results are shown in Figure 5.7. The cathode side of cell 8 showed high water accumulation rates compared to other cells. Cathode side of cell 7 also showed a higher trend than the rest of the six cells. In contrast to the cathode side, cell 8 anode showed much lower water accumulation than rest of the cells. Disregarding


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the effect of the 335 h emergency stop the water accumulation rates stayed sufficiently stable during the measurements.

Figure 5.7. Exhaust water accumulation from anode (A) and cathode (C) of each cell of the MSC6 during the long-term test.

As the thickness of GDL materials varied between samples, the compression pressures might have had major differences. Some differences also arose from tolerance errors between the graphite bipolar plates and the assembly process, which mostly explained the varying performance between cells assembled with identical GDLs. As the water accumulation rates suggested, there were also notable differences on humidity conditions between the cells. Water condensation was evident on the adjacent cells. Furthermore, water partially filling any of the gas flow channels also affected the pressure drop and complicated the flow adjustment between the cells. Although the compression and flow conditions differed between cells, these did not show any notable effect on the operation voltages and polarization scans. However, the impedance spectra measured at the beginning of the test showed more clear differences between individual cells.

5.1.3 Single cell testing

The lengthy multisinglecell testing raised a need for quick single cell characterization method. With conventional methods, the break-in done with each unused MEA takes a considerable amount of time compared to the actual characterization tests done afterwards. The characterization procedure only included simple polarization scans, impedance spectroscopy and humidity sweeps,


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which should take no more than 8 hours compared to the break-in which might require 24 hours. Some literature research was done to find suitable break-in procedure to speed up the characterization process. A break-in cycle called “air break-in” was tested. The cycle consisted of heating the fuel cell up to operating temperature and cutting the air supply for small periods of time while the cell was under load conditions. This cycle was repeated until the cell performance was stabilized. The test was run at 78 °C with hydrogen flow rate of 0.2 nlpm and air flow rate of 0.5 nlpm. Both gases were fully humidified. Current density was kept at constant 100 mA/ cm2 during the test, except for the polarization scans. The initial results showed the “air break-in” could be done in less than 5 hours. This would reduce the total duration of characterization process so that one batch could be tested in less than 24 hours. After 36 cycles, the performance seemed to have set to a constant level shown by the polarization curves and impedance data presented in Figure 5.8 and Figure 5.9, respectively. The testing also included five polarization scans, which also have a contribution to the break-in.

Figure 5.8. Polarization curves measured during the “air break-in” test at 78 °C. The voltage was measured at 100 mA/cm2 intervals from 500 mA/cm2 to 100 mA/cm2.

50 100 150 200 250 300 350 400 450 500 5500.55

0.6

0.65

0.7

0.75

0.8

Current density (mA/cm2)

Cel

l vol

tage

(V)

Polarisation curves measured during the break-in cycles

Before cyclingAfter 1 cycleAfter 6 cyclesAfter 16 cyclesAfter 36 cycles


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Figure 5.9. Impedance spectra measured during the “air break-in” test at 78 °C temperature and 200 mA/cm2 current density from 10 kHz to 0.5 Hz with 10 points per decade. Voltage perturbation amplitude was 10 mV.

5.2 Ex-situ characterization

5.2.1 Mechanical properties

5.2.1.1 Compressibility

5.2.1.1.1 Dynamic mechanical analysis Compressive behavior of Sigracet GDL 35 DC was analyzed by DMA. The first test series were made with the constant minimum dynamic force value of 0.01 N and only the static force was increased gradually. The stress-strain curves for the samples were constructed from the measured raw data values for static force and corresponding displacement values. The average of the results from these measurements was similar to a reference static compression test conducted elsewhere, but the dispersion between parallel samples was quite large. Compression tests were continued with different measurement technique. Tests were made with gradually increasing both dynamic and static force values and the corresponding displacement values were measured. Results from these four parallel tests are presented in Figure 5.10. In the same figure the most representative measurement from the previous static force tests conducted with constant dynamic force of 0.01 N and the reference measurement are also presented.

0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02 0.022 0.0240

1

2

3

4

5

6x 10

-3

Real(Z) (Ohm)

-Imag

inar

y(Z)

(Ohm

)

Impedance spectra measured during the break-in cycles

Before cyclingAfter 1 cycleAfter 6 cyclesAfter 16 cyclesAfter 36 cycles


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Figure 5.10. Compressibility of Sigracet 35 DC determined by DMA. Tests 1 through 4 represent the measurements made by increasing both dynamic and static force, with measurement head of either 4 mm (tests 1 & 2) or 5 mm (tests 3 & 4) in diameter. A static DMA test (only static force increased) and another static test (conducted at Aalto University by Mikkola as reported in [13]) are shown for reference.

One of the biggest challenges in the DMA compression measurements was the tightening of the sample to the sample holder. Excessive sample pre-tightening generated easily distortion in results. In addition, the maximum combined force of the DMA device was 40 N which limited the sample size. Samples in these tests were 5 mm and 4 mm in diameter but it seemed that 4 mm samples were too small for reliable results.

5.2.1.1.2 Kawabata evaluation system KES was another method applied for evaluating the compressive behavior of carbon paper GDLs. The results of the measurements are listed in Table 5.2.

Table 5.2. Compression test data of Sigracet GDL samples 34 BC, 35 BC, 35 CC and 35 DC. LC = linearity of compression-thickness curve, WC = compressional energy, RC = compressional resilience, T0 = initial thickness, TM = thickness in compression and TSGL = thickness specification by SGL.

Sample LC

WC [cgcm/cm2] RC [%]

T0 [mm]

TM [mm] (4.9 kPa)

TSGL[mm] (35 kPa)

34 BC 0.350 0.100 79.707 0.383 0.254 0.315 35 BC 0.389 0.096 76.200 0.397 0.278 0.325 35 CC 0.498 0.073 68.089 0.347 0.270 n.a. 35 DC 0.517 0.067 65.939 0.355 0.299 n.a.

The compressive pressure of 4.9 kPa is rather low in comparison to the typical PEMFC stack compression of 1 MPa. Moreover, there is some discrepancy between the measured values under compression and the values reported by SGL under a higher compression of 35 kPa.

0,00

0,50

1,00

1,50

2,00

2,50

3,00

0 5 10 15 20 25 30 35

Compression (%)

Stre

ss (M

Pa)

test (1) dynamic d=5mmtest (2) dynamic d=5mmtest (3) dynamic d=4mmtest (4) dynamic d=4mmPrevious static test d=5mm

reference measurement


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5.2.1.2 Tensile properties Stress-strain curves and the calculated average tensile properties of Sigracet carbon paper samples are presented in Figure 5.11 and Table 5.3, respectively. Two parallel measurements were done in both long and cross direction.

Figure 5.11. Stress-strain curves of of Sigracet GDL samples 34 BC, 35 BC, 35 CC, and 35 DC in long (L) and cross (C) direction.

It can be seen that the strength in long direction is higher than that of cross direction of the sample. This may indicate that there is anisotropy in the orientation of the fibers between the two perpendicular directions.

Table 5.3. Results of tensile tests of Sigracet samples 34 BC, 35 BC, 35 CC, and 35 DC: Averages of Young’s modulus [GPa], force at break [N], strain at break [%], and modulus at break [GPa].

Sample

Young’s modulus

[GPa] Force at break

[N] Strain at break

[%]

Modulus at break [GPa]

34 BC 3.0 83 1.2 % 1.1 35 BC 2.4 54 0.8 % 0.9 35 CC 2.9 63 1.0 % 0.9 35 DC 2.9 67 1.0 % 0.9

No significant differences in the average tensile properties between the samples were observed.

5.2.1.3 Bending Bending strength of Sigracet 34 BC, 35 BC, 35 CC and 35 DC was measured, and the results are collected in Table 5.4. It was observed that the bending strength to the long-direction was higher compared to that of the cross-direction on both sides. Bending strength of Sigracet 35 CC and 35 DC was little higher than the


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strength of other samples when the MPL side was the rounded surface in bending. The power cell of the tensile machine (25 kN) was too big for these measurements and therefore the results are not perfectly reliable.

Table 5.4. Bending stiffness data of Sigracet GDL samples 34 BC, 35 BC, 35 CC, 35 DC. MPL/substrate side = the side of the sample that has been rounded in L = long-direction or C = cross-direction.

5.2.2 Porosity

5.2.2.1 Gas adsorption method Microporosity (0.1-2.0 nm) and mesoporosity (2.0-70 nm) of the Sigracet samples was measured by BET porosity method. BET porosity test data are given in Table 5.5.

Table 5.5. BET porosity test data of Sigracet GDL samples.

Sample

Surface area [m2/g]

Micropore volume l/g]

Mesopore size [nm]

Mesopore volume

l/g] 34 BC 7.88 3.8 < 68.6 21.1 35 BC 9.38 4.6 < 69.9 24.2 35 CC 10.20 4.9 < 80.0 29.1 35 DC 10.95 4.9 < 74.6 26.7

The microporosity is attributed to the internal porosity of the carbon black used in the microporous contact layer and mesoporosity to the porosity between the carbon black particles in the same layer. As the MPL was identical in all cases, there were no large differences in the results. The MPL in Sigracet 34 BC is slightly denser than in Sigracet 35 grades, which reflects the differences in the substrate porosity.

5.2.2.2 Mercury intrusion porosimetry MIP was performed on Carbel in order to obtain information about the porosity of the sample. The total pore volume was measured to be 1620 mm3/g and total pore surface area 9.28 m2/g. The pore size distribution graph is presented in Figure 5.12.

Sample Bending stiffness (Nm·m)

MPL side, L Substrate side, L MPL side, C Substrate side, C 34 BC 5 8 3 4 35 BC 5 6 3 4 35 CC 7 4 6 3 35 DC 6 6 4 4


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Figure 5.12. Pore size distribution of Carbel determined by MIP.

About half of the total pore volume resulted from pores with diameter between 30 and 100 m. Pore sizes from about 4 m to 100 m were attributed to the porosity of the substrate. The smallest observed diameter was approximately 0.05 m, which was related to the porosity between the carbon black particles in the MPL. MIP is therefore not suitable for studying the microporosity of GDLs.

5.2.3 Permeability

5.2.3.1 Gurley method The results of the Gurley air permeability measurements are presented by Table 5.6. For comparison, the values reported by the Sigracet manufacturer (SGL Carbon) are shown for the samples for which it was published.

Table 5.6. The results of the Gurley air permeability tests. 35 AA was too permeable for the range of the instrument.

Air permeability [cm3/cm2·s] Pressure 35 AA 34 BC 35 BC 35 CC 35 DC [Pa]

VTT - 0.7 3.8 5.7 9.6 1296 SGL n.a. 0.4 1.5 n.a. n.a. 304

The 35 series is known to be more porous than the 34 series. However, the air permeability seemed to increase in VTT measurements with every sample, and not only when going from 34 BC to 35 series. It was therefore suggested that the increase in PTFE content increased the permeability in these conditions, possibly because PTFE may have helped to keep the structure more open under pressure.


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5.2.3.2 Karl Schroder tester Air and water vapor permeability data are given in Table 5.7. The measured air permeabilities are higher than the values reported by the producer based on the Gurley method.

Table 5.7. Air and water vapor permeability data of Sigracet GDL samples 34 BC, 35 BC, 35 CC and 35 DC. Air permeability specifications according to the manufacturer are shown in parentheses.

Figure 5.13 illustrates graphically the air permeability values measured with different techniques. The difference between the values determined at TUT, and those determined at VTT and given by Sigracet, was obvious. It can be concluded that the method used at TUT, which is mainly designed for textiles, is not suitable for this kind of samples, as it does not give almost any difference between the samples.

Figure 5.13. Air permeability of Sigracet samples determined by VTT, TUT and the manufacturer. Note that the measurements are not totally comparable as the used pressure is different in each technique.

0,0

2,0

4,0

6,0

8,0

10,0

12,0

34BC 35BC 35CC 35DC

cm3 /

cm2 s

TUT

VTT (Gurley)

Sigracet (Gurley)

Sample Air permeability Water vapor permeability

[l/min] [mm/s] [l/cm2*s] [g/m²*Pa*h] [g/m²*24h]

34 BC 3 25 0.0025

(0.00035) 0.585 23884

35 BC 4 31 0.0031

(0.0015) 0.702 27504 35 CC 4 33 0.0033 0.748 28794 35 DC 4 35 0.0036 0.637 25492


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5.2.4 PTFE content

5.2.4.1 Energy dispersive elemental analysis The fluorine contents of sample surfaces measured by EDS are given in Table 5.8.

Table 5.8. Fluorine content (weight-% and error) measured on both sides of Sigracet GDL BC, 35 BC, 35 CC and 35 DC by EDS. samples 34

Sample MPL Substrate

Measured fluorine wt%

Specified PTFE wt%

Measured fluorine wt%

Specified PTFE wt%

34 BC 21.02 ± 0.44 10 30.71 ± 0.66 5 35 BC 21.36 ± 0.50 10 34.81 ± 0.51 5 35 CC 22.02 ± 0.52 10 31.71 ± 0.64 10 35 DC 22.23 ± 0.54 10 36.75 ± 0.57 20

No clear correlation between the specified PTFE content and the measured fluorine content was observed. It is probable that the fluorine content was too high and the detection limit was saturated.

5.2.4.2 Sessile drop method Surface energies of both the Sigracet and Carbel samples were determined by contact angle measurements. The results are presented by Table 5.9 and Figure 5.14.

Table 5.9. Results of the repeated surface energy measurements.

Sample

Substrate side Dispersion Polar Total

Test 1 Test 2 Test 1 Test 2 Test 1 Test 2 34 BC 5.64 3.99 1.72 1.25 7.35 5.24 35 BC 3.36 4.89 0.086 1.20 3.45 6.09 35 CC 5.21 6.05 0.845 1.74 6.05 7.79 35 DC 5.15 4.48 0.401 1.22 5.55 5.70 Carbel 2.28 - 0.05 - 2.33 -

Sample MPL side

Dispersion Polar Total Test 1 Test 2 Test 1 Test 2 Test 1 Test 2

34 BC 1.28 0.36 0.000 0.013 1.28 0.37 35 BC 1.12 0.31 0.002 0.059 1.12 0.37 35 CC 0.85 0.38 0.000 0.049 0.85 0.43 35 DC 1.16 0.38 0.116 0.028 1.27 0.40 Carbel 0.84 - 0.013 - 0.86 -


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Figure 5.14. Total surface energies with standard deviation. Samples are arranged by the growing PTFE content in the substrate side. (5 wt% for BC, 10 wt% for CC and 20 wt% for DC. PTFE content in Carbel not known.)

Total surface energy of the MPL side was approximately same for all the samples, which was expected since the composition of the MPL was same at least in all Sigracet samples. Another remark done was that Carbel had the lowest total surface energy on the substrate side. This could indicate that the PTFE content was higher in Carbel than in Sigracet samples. The total surface energy of the substrate side was seen to decrease when the amount of PTFE increased, which was mainly due to the decrease in polar energy, which indicated that the surface was more hydrophobic. Sample 35 BC however seemed to form an exception in this case. Therefore another trial of the surface energy measurements was done in order to verify or disprove the deviation. The results from the repeated measurements differed greatly from the previous ones when surface energy seemed to decrease with increasing PTFE content. This indicated that the static contact angle method was not repeatable with these samples. The contact angle evolution measured at TUT as a function of time is presented in the case of Sigracet 34 BC MPL side in Figure 5.14. Measuring proved quite difficult but some results were obtained with distilled water. It was seen that a drop of water needs at least 1 s to stabilize on the surface. Measurements with solvents were unsuccessful and so it was not possible to calculate surface energy values. Contact angle values of different Sigracet samples measured with water were near to each other.

34 BC

35 BC

35 CC35 DC

Carbel

34 BC 35 BC35 CC

35 DCCarbel

34 BC

35 BC

35 CC

35 DC

34 BC 35 BC 35 CC 35 DC0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

9,0

Surf

ace

ener

gy (m

N/m

)

Sample

Surface energies determined by the OWRK method

Test 1, substratesideTest 1, MPL side

Test 2, substratesideTest 2, MPL side


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Figure 5.15. Contact angle evolution of water droplet on the MPL surface of Sigracet 34 BC.

The poor applicability of this method was probably due to the coarse surface of the GDLs: the surface roughness affected the contact angle more than the chemical content of the surface. [14]

5.2.4.3 Thermogravimetric analysis TGA was performed on four Sigracet samples, where the PTFE content is gradually increasing, and to pure PTFE. The results are shown in Figure 5.16.

Figure 5.16. Results of the TGA for Sigracet GDL 35 AA, 35 BA, 35 CC, 35 DC and pure PTFE run under nitrogen atmosphere.

As carbon should be stable in nitrogen atmosphere, the mass loss occurring in GDLs samples was attributed to the pyrolysis of PTFE. This was verified by the TGA curve of pure PTFE, where the actual PTFE pyrolysis occurred at same temperature range (550-600 °C) where the decrease in mass was visible in GDL


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samples. The evaporation of solvents around 100 °C was also visible in the PTFE curve. It could be seen that the mass losses did not correspond to the PTFE content of the samples. For instance, the mass loss in 35 DC was over 25 %, whereas the actual PTFE content in 35 DC was less than 20 % (20 % in substrate and 10 % in MPL). The behavior was similar in all the cases: the observed amount of PTFE pyrolyzed was greater than the actual PTFE content in the sample. This could only be explained by the fact that some carbon was either flown away with the nitrogen flow during the test, or carbon was corroded in the presence of fluorine gas or some other reactive decomposition product of PTFE.

5.2.5 Electrical conductivity

5.2.5.1 Through-plane conductivity Through-plane conductivity of the samples was measured both at VTT and at TUT in similar conditions. The results are provided by Table 5.10.

Table 5.10. Results of the through-plane resistivity measurements.

Sample Resistivity (m •cm2)

VTT (1 MPa) TUT (0.9 MPa) 35 AA 5.5 - 35 BA 7.6 - 34 BC 10.4 11.5 35 BC 10.7 11.3 35 CC 11.7 12.6 35 DC 13.2 17.7 Carbel 9.4 -

The resistivity values measured at VTT are slightly lower than those measured at TUT, which was probably due to the compaction pressure difference. The resistivity increased with increasing PTFE content.

5.2.5.2 In-plane conductivity In-plane resistivity (or volume resistivity) measurements were conducted on both Sigracet and Carbel samples. The results are presented by Table 5.11. Each value shown is the mean value of at least four parallel measurements.

Table 5.11. Results of the volume resistivity measurements.

Volume resistivity [m ·m]

Sample MPL side Substrate side 34 BC 0.14 0.12 35 BC 0.15 0.18 35 CC 0.22 0.20 35 DC 0.24 0.22 Carbel 0.13 0.10

On both sides the resistivity values seemed to grow with the increasing PTFE content of the substrate, whereas the resistivity of Carbel was the lowest. The


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reason why the resistivity was increasing on the MPL side as well regardless of the PTFE content, is that the current does not flow only on the surface but deeper, in this case through the substrate side as well. Therefore the values given for the MPL side do not only describe the resistivity of the microporous layer but also the substrate side as the microporous layer is much thinner than the substrate. All in all, these results were in the same order as the magnitude as the values obtained in Marapoke 1st phase by Aalto University: 0.23 m ·m for 35 BC and 0.17 m ·m for Carbel. However, the side on which these values were measured was not reported.

5.2.6 Thermal conductivity

The HotDisk measurement system was verified to be quite suitable for measuring carbon papers. However, for the measurements it was essential to know the thickness of the carbon paper sample under pressure. An estimation for this value was only available for Sigracet GDL 35 BC from the earlier DMA stress-strain curves, but the accuracy of this estimation was unclear. The preliminary result, 0.14 W/m·K for Sigracet GDL 35 BC, was smaller or about the same compared to corresponding literature values [15], [16].

5.2.7 Structure

5.2.7.1 3D profilometry 3D profilometry was used to characterize the surface roughness of Sigracet samples. Figure 5.17 shows the results for sample 35 BC as an example.

Figure 5.17. 3D profilometry results for sample Sigracet 35BC. Pa refers to “primary profile”, which is determined as an ”average, unstrained roughness value on the measured surface area”. The red line in the upper left corner picture marks the line profile.

The profilometry figures show clearly the mud-cracking on the MPL surface. The primary profiles, or roughness values, based on area and line calculations are summarized in Figure 5.18.


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Figure 5.18. Primary profile values of Sigracet samples 34 BC, 35 BC, 35 CC and 35 DC based on 3D profilometry.

There seemed to be a difference between the 34 and 35 series in roughness values of area based calculations, which probably was related to the porosity differences between these grades (34 has lower porosity than 35). Line based roughness values seemed randomly dispersed and depended heavily on the presence or absence of cracks under the line in question.

5.2.7.2 Atomic force microscopy Topography and phase imaging by non-contact tapping mode of AFM was used in order to examine the MPL side of the GDLs. Figure 5.19 shows an example of an image where the separate 3D topography and phase images were put on top of each other.

Figure 5.19. 5 x 5 m2 AFM phase image on top of 3D topography image of the MPL surface of Carbel GDL.

The white parts of the image are areas where the AFM tip has not fully tracked the surface due to sudden changes in the altitude profile of the sample. The phase

0

2

4

6

8

10

12

14

34BC 35BC 35CC 35DC

Prim

ary

prof

ile,

m

Sample

area

line


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images revealed that there were variations in the surface structure of the MPL, but the relations of the different species could not be solved. Conductive AFM imaging, run in contact mode, was not very suitable for the MPL side, which contained loose carbon black sticking to the probing tip preventing the tip from tracking the surface properly. On the other hand, the substrate side was so rough, that only individual fibers could be imagined. An example of an iAFM image is presented in Figure 5.20.

Figure 5.20. 1 × 1 m2 3D topography image from a single fiber on the substrate side of the sample 35 BC. The conductivity map is superposed on the topography image showing the conductive areas in white.

6 Conclusions

Complexity of the MSC setup required a great deal of adjusting and reconstruction, but finally a properly functioning setup was constructed. Long-term in-situ testing of GDLs in a MSC setup provided a large amount of data. Evolution of cell voltage, polarization curves, impedance data in different conditions and exhaust water accumulation were all collected in order to use them as secondary information related to the GDL degradation. The results show the importance of the MPL in the electric contact between the GDL and the catalyst layers as well as in water management. No degradation of the GDLs with the MPL could be observed for the catalyst layers or membranes were degrading much faster. The catalyst degradation could be due to the high humidity and temperature used in the measurements. More information about the degradation mechanisms could be acquired by equivalent circuit modeling of the EIS spectra but this was beyond the scope of this study. No clear correlation between the PTFE content in the GDL substrate and the degradation rate was observed. On the other hand, increasing the PTFE content from 5 to 20 wt% did not show any negative effect under the MSC specific operating conditions either. Regarding the ex-situ characterization of GDLs, it was shown that to some extent it is possible to characterize GDLs with methods commonly used in materials


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characterization. The important thing to remember is that GDLs are anisotropic due to continuous manufacturing methods, which means that many of their properties are direction dependent. Post-mortem analysis of GDLs alone is troublesome because detaching the GDL from the membrane after cell operation without damaging the MPL is difficult. Therefore artificial ageing methods for GDLs should be developed. Evaluated ex-situ characterization methods are presented in Table 6.1. A deeper insight into the both in-situ and ex-situ characterization methods is presented in technical report [17] based on literature review.

Table 6.1. Evaluated ex-situ characterization methods and their applicability for GDL characterization.

Property Method Applicability

Compressibility

DMA Maximum force of the instrument too low, static measurement is more reliable

Kawabata Maximum force is all too low, not applicable

Tensile properties

Universal testing machine

OK

Bending Universal testing machine

OK if right size of force cell is available

Porosity BET Measures micro- and mesoporosity of carbon black, not applicable for the substrate

MIP OK

Air permeability Gurley OK, the instrument measurement range must correspond sample properties

Karl Schroder Not applicable Water vapor permeability Karl Schroder No better methods reported in the

literature

PTFE content

EDS Not applicable

Sessile drop Not applicable, but gives a rough estimate for hydrophobicity

TGA Further development of test conditions needed in order to avoid carbon loss.

iAFM Not applicable, too small sample area

Electrical conductivity

Gold plated electrodes under compression OK for through plane measurement

4-point method OK for in plane measurement

Thermal conductivity HotDisk

OK for through plane measurement if sample thickness under compression can be measured accurately

Surface image and roughness

3D profilometer OK

AFM Too small sample area to be representative

Determining the mechanical properties of GDLs is often complicated by the fact that many testing devices are designed for metals, plastics or fabrics, and are therefore too robust for GDL characterization. Despite the lack of full applicability of these testers, it may still be possible to gather some qualitative


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information. When measuring permeability, it has to be noted that GDLs are generally designed to be very permeable, and therefore they may be out of the range of some commonly used testing procedures. On the other hand, porosity measurements, especially when BET analysis was combined with MIP, can give information of the whole pore size scale from the smallest pores of the MPL to the largest pores of the substrate. It was also noted that electrical conductivity measurements by common four-point probe configuration combined with house-made measuring devices proved to be reliable. In some cases the surface roughness of GDLs complicated or even prevented gathering reliable results, as was the case in AFM imaging and contact angle measurements, respectively. 3D laser profilometry was identified as a quick and easy technique for imaging the surface profile of GDLs, but the usefulness of this information remained quite low. The determination of PTFE content in GDLs proved challenging as none of the used methods gave reliable information. The sessile drop method suffered from surface roughness, whereas with EDS the problem presumable lay in the exceeded detection limit. The actual reason for the failing of the TGA was not verified, but some ideas for further development were identified. For instance, the analysis could be repeated with the temperature rising more slowly, which would slow down the pyrolysis reactions and therefore the inert gas atmosphere could protect the sample better. Alternatively, the inert gas flow rate could be altered in order to see if it has any effect on the observed mass losses. Both in-situ and ex-situ characterization methods proved to be much more complicated and tedious than originally expected. Therefore no real correlations between ex-situ properties and in-situ performance could be found out.

7 Summary

Gas diffusion layers for polymer exchange membrane fuel cells were studied. Various ex-situ characterization methods were evaluated, and in-situ characterization in multisinglecell setup was developed resulting in one long-term test of about 1200 hours. A technology follow-up related to PEMFCs and GDLs resulted in a technical report of GDL characterization methods, which later on was used as groundwork for a journal article to be published in Journal of Power Sources.


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References

[1] L. Cindrella et al., “Gas diffusion layer for proton exchange membrane fuel cells - A review,” Journal of Power Sources, vol. 194, no. 1, pp. 146-160, Oct. 2009.

[2] F. A. de Bruijn, V. A. T. Dam, and G. J. M. Janssen, “Review: Durability and Degradation Issues of PEM Fuel Cell Components,” Fuel Cells, vol. 8, no. 1, pp. 3-22, Feb. 2008.

[3] G. Chen, H. Zhang, H. Ma, and H. Zhong, “Electrochemical durability of gas diffusion layer under simulated proton exchange membrane fuel cell conditions,” International Journal of Hydrogen Energy, vol. 34, no. 19, pp. 8185-8192, Oct. 2009.

[4] M. Mathias, J. Roth, J. Fleming, W. Lehnert, and A. Opel, “Diffusion media materials and characterisation,” in Handbook of Fuel Cells, Fuel Cell Technology and Applications: Part 1, vol. 3, W. Vielstich, A. Lamm, and H. A. Gasteiger, Eds. West Sussex: John Wiley & Sons, Ltd, 2003, pp. 1-21.

[5] “CeTech - Carbon Paper: Sheet type,” 2007. [Online]. Available: http://www.ce-tech.com.tw/english/GDL-03.html. [Accessed: 16-Nov-2011].

[6] S. Park, J.-W. Lee, and B. N. Popov, “A review of gas diffusion layer in PEM fuel cells: Materials and designs,” International Journal of Hydrogen Energy, vol. 37, no. 7, pp. 5850-5865, Apr. 2012.

[7] J. Wu et al., “A review of PEM fuel cell durability: Degradation mechanisms and mitigation strategies,” Journal of Power Sources, vol. 184, no. 1, pp. 104-119, Sep. 2008.

[8] A. Arvay et al., “Characterization techniques for gas diffusion layers for proton exchange membrane fuel cells – a review,” Journal of Power Sources, 2012.

[9] S. Kawabata, The standardization and analysis of hand evaluation, 2nd ed. Osaka: Textile Machinery Society of Japan, 1980, p. 97.

[10] M. Parfitt, J. C. Vickerman, C. M. Carr, N. Ince, and P. Knight, “Surface analysis of softened paper by time-of-flight secondary ion mass spectrometry (ToF-SIMS) and the Kawabata evaluation system,” vol. 8, pp. 2171-2177, 2003.

[11] S. M. Sze and K. K. Ng, Physics of semiconductor devices, Third Edit. Hoboken, New Jersey: John Wiley & Sons, 2007, pp. 30-31.

[12] M. S. Ismail, T. Damjanovic, D. B. Ingham, M. Pourkashanian, and A. Westwood, “Effect of polytetrafluoroethylene-treatment and microporous layer-coating on the electrical conductivity of gas diffusion layers used in proton exchange membrane fuel cells,” Journal of Power Sources, vol. 195, no. 9, pp. 2700-2708, May 2010.

[13] P. Kauranen et al., “Advanced Material Solutions for PEM Fuel Cells – Final Report,” Tampere, 2010.


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[14] V. Gurau, M. J. Bluemle, E. S. De Castro, Y.-M. Tsou, J. A. Mann, and T. A. Zawodzinski, “Characterization of transport properties in gas diffusion layers for proton exchange membrane fuel cells,” Journal of Power Sources, vol. 160, no. 2, pp. 1156-1162, Oct. 2006.

[15] A. Radhakrishnan, “Thermal Conductivity Measurement of Gas Diffusion Layer Used in PEMFC,” Kate Gleason College of Engineering / Rochester Institute of Technology, 2009.

[16] A. Pfrang, D. Veyret, F. Sieker, and G. Tsotridis, “X-ray computed tomography of gas diffusion layers of PEM fuel cells : Calculation of thermal conductivity,” International Journal of Hydrogen Energy, vol. 35, no. 8, pp. 3751-3757, 2010.

[17] E. Yli-Rantala and P. Koski, “Characterization of gas diffusion layers for polymer electrolyte fuel cells,” Tampere, 2012.

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