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Electrochemical measurements
The electrochemical measurements were conducted using an Autolab PGSTAT20. Fresh electrolyte (0.1 M H2SO4) was daily prepared from ultrapure water (MQ grade, 18.2 MΩ cm, 1-3 ppb total organic compound (TOC)) and 96 wt. % H2SO4 (Suprapur, Merck). For all electrochemical experiments, the electrolyte was purged with argon (Ar, > 99.99 %, Messer).
In Chapter III, two separated four-electrode electrochemical cells were used, a “characterization cell” and a “degradation cell” both thermostated at T = 330 K. A homemade rotating disk electrode tip was used as working electrode. A large-area Pt foil was used as counter-electrode for the studies of sub-chapter 3.4, 3.5 while a carbon monolith was used as counter-electrode for the study of sub-chapter 3.6. A mercury sulphate electrode Hg|Hg2SO4|K2SO4 (saturated, aqueous) – connected to the cell via a Luggin capillary as a reference electrode. This reference electrode was calibrated periodically by measuring the potential difference with a reversible hydrogen electrode (RHE), the latter being systematically 0.72 V vs. RHE. All the potentials reported in this study were referenced on the RHE scale. A Pt wire connected to the reference electrode was used to filter the high frequency electrical noise and to avoid disturbing the low frequency electrical measurements. More details on the dual-reference system used in this work can be found in Ref. [1].
In Chapter IV, two separated four-electrode electrochemical cells were used, a “characterization cell” and a “degradation cell” both thermostated at T = 330 K. A homemade rotating disk electrode tip was used as working electrode, a large-area Pt foil as counter-electrode and a mercury sulphate electrode Hg|Hg2SO4|K2SO4 (saturated, aqueous) – connected to the cell via a Luggin capillary as a reference electrode.
Preparation of the catalytic inks and of the porous catalytic layers
The preparation of reproducible porous catalytic layers is extremely important in the field of electrocatalysis of PEMFC materials, the interested reader is referred to the excellent review by Kocha et al. on this issue [2].
Preparation of the catalytic inks
Suspensions composed of a 5 wt. % Nafion® solution (Aldrich), MQ-grade water, 20 µL of isopropanol and 5.0 mg of Pt/C powder were ultrasonically treated for 15 minutes and used as catalytic inks. The ionomer/electrocatalyst (I/C) ratio was 0.5. The 40 wt. % Pt/HSAC electrocatalyst inks have a concentration of 0.735 gPt/C L−1. The inks were used over a period of 1.5 month, such “expiring date” being fixed by the changes in chemistry and structure of the Pt/C electrocatalysts over time (see Ref. [3] for more details).
Preparation of the porous catalytic layers
Home-made rotating disk electrodes (RDE) made of a glassy carbon disk (Sigradur, 0.196 cm²) embedded in a Kel-F cylinder were firstly manually polished with a 1 µm diamond paste (Mecaprex, Presi) in a “figure eight” pattern for 5 minutes until a mirror finish was obtained. The polished electrode was then rinsed with ultrapure water, and subsequently sonicated in acetone, ethanol and MQ-water solutions to remove the excess of diamond paste (15 minutes for each solution). The electrodes were then stored over-night in a beaker filled with MQ-water sealed with Parafilm®, and placed inside the laboratory cabinet to prevent contamination by air impurities.
On the experiment day, the rotating disk electrodes were air-dried in an oven at T = 383 K for 6 minutes. Meanwhile, the electrocatalyst ink was sonicated to ensure homogenization of the catalyst’s ink. An aliquot of 20 µL was then deposited onto the glassy carbon disk of the freshly polished RDE and the ink was dried for 5 minutes in air at T = 383 K to ensure evaporation of the water and the solvents of the Nafion solution. This resulted in a so-called “porous” RDE. In Chapter IV a second aliquot of 20 µL was added yielding a final Pt loading of 11.76gPt (2 × 5.88gPt). The working electrode was always immersed/withdrawn into/from the electrochemical cell at controlled electrode potential E = 0.40 V vs. RHE.
Electrochemical characterization
Before any electrocatalytic measurement, ten cyclic voltammograms at v = 20 mV s-1 followed by another one at v =100 mV s-1 were firstly recorded to obtain the characteristic voltammetric response of the Pt/C electrocatalysts between 0.05 and 1.23 V vs. RHE. Secondly, the electrochemically active surface area (ECSA) was measured using the coulometry required to desorb a monolayer of under-potentially adsorbed H or electrooxidize a monolayer of adsorbed carbon monoxide (COad stripping voltammograms). The carbon monoxide (CO) saturation coverage was established by bubbling CO for 6 min and purging the solution with Ar for 45 min, while keeping the electrode potential at E = 0.10 V vs. RHE. It was assumed that the electrooxidation of an adsorbed CO monolayer required 420C per cm2 of Pt.
Accelerated stress tests (ASTs)
Different ASTs were designed to isolate the effect of specific parameters on the degradation mechanisms and the durability of the Pt/C electrocatalysts:
To test the effect of the potential range and gas atmosphere in the degradation of the electrocatalysts, ASTs defined by the FCCJ organization were performed [4]. The tests consist of two protocols (Figure II – 1):
– Load-cycle protocol: square potential ramp between 0.60 V – 1.00 V vs. RHE with a period of 6 s/cycle, which mimic the potential range experienced during PEMFC operation under steady-state;
– Start-up/shutdown protocol: square potential ramp between 1.00 V –1.50 V vs. RHE with a period of 6 s/cycle, which mimic the potential range experienced during PEMFC operation under start/stop cycles;
The ASTs protocols are performed in the “degradation cell”, during 5000 cycles with 0.1 M H2SO4 as electrolyte, under neutral (argon) or oxidising (oxygen) atmosphere.
The accelerated stress tests performed in this chapter consisted of potentiostatic measurements performed at E = 0.40, 0.50, 0.60, 0.70, 0.80, 0.90, 1.00, 1.20, 1.30, or 1.40 V vs. RHE for 96 hours. The changes in ECSA were monitored via COad stripping coulometry (after 6, 24, 48, 72 and 96 h) in the characterization cell. Before the COad stripping, five cyclic voltammograms at v = 100 mV s-1 and a cyclic voltammogram at v = 20 mV s-1 were recorded to characterize the surface reactivity of the Pt/HSAC electrocatalyst.
PEMFC testing
A 110-cell PEMFC stack using Pt/C at the anode and at the cathode was operated at constant current density (j = 0.25 A cm-2) for t = 12,860 h at T = 333 to 338 K by Axane (Isère, France). The electrocatalytic materials, the chemical nature of the ionomer contained in the catalytic layers and the proton-exchange membrane are proprietary and cannot be described. The catalytic layers were catalyst-coated membranes (geometric area 86 cm2). The cathode was fed with humidified air (60 to 75% relative humidity (RH), close to the atmospheric pressure, gas stoichiometry of 2.5), and the anode was fuelled with dry pure dihydrogen (0 % RH, 1.35 bar abs., dead-end mode, corresponding to an average H2 stoichiometry of ca. 1.1). Air and H2 purges were performed intermittently to remove the water produced at the cathode and the water and nitrogen that accumulate at the anode, respectively. Practically speaking, the air RH was fixed at the gas inlet by an adequate choice of the humidifier and cell temperature, and the air stoichiometry was doubled intermittently to allow air purges. The frequency and the duration of the air purge are proprietary and cannot be mentioned. The stack was disassembled at the end of life, and used for physical, chemical and electrochemical characterization. All the data presented in this study were obtained for electrocatalysts sampled in different regions of the cathode: cathode inlet/outlet and regions corresponding to segment 5 and segment 8 of LEMTAs segmented cell (see Chapter V).
Ex situ preparation of the fresh/aged MEAs cathode electrocatalysts
For the post-mortem analysis of the cathode electrocatalyst of a catalyst-coated backing (CCB) MEA, it is not possible to detach the gas diffusion-layer (GDL) from the proton-exchange membrane (PEM) without affecting the electrode structure. Small pieces of the MEA were then cut with a punch, and some droplets of ethanol were added onto the surface 43 of the GDL (anode and cathode) to facilitate its detachment from the PEM. It was possible to observe that a fraction of electrocatalyst remained on the PEM surface (middle circle in step 6 of Figure II – 2), but most of the catalytic powder was attached to the GDL. For TEM imaging, the remaining electrocatalyst present in the Nafion® membrane (cathode side) was scraped with a metallic spatula, dispersed in MQ-grade water and deposited on a TEM grid. The cathode side of the GDL, which was covered with the fresh/aged catalytic powder, was used for Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) measurements without any further treatment. The reliability of such procedure is shown in Figure II – 3, where a Raman spectra of aged Pt/C on the GDL and on the PEM are presented, revealing identical features.
Characterization of the fresh and aged electrocatalysts
Physical techniques
Electron microscopes use a beam of highly energetic electrons to examine nanometre scale objects on a very fine scale. This examination provides information about the topography (surface features of an object), the morphology (shape and size of the particles making up the object), the composition (the elements and compounds that the object is composed of and the relative amounts of them) and the crystallographic information (how the atoms are arranged in the object). The advantage of such microscopes is the use of electrons beam (shorter wavelength than white light), which provides them the possibility to distinguish “atomic” features.
Most electron microscopy rely on the same principle: (i) firstly, a beam of electrons is formed in high vacuum (by electron guns), (ii) secondly the electron beam is focused using metal apertures and magnetic lenses into a thin, focused, monochromatic beam, (iii) thirdly the sample is irradiated by the beam and interactions occur inside the irradiated sample, affecting the electron beam and finally (iv) these interactions are detected and transformed into an image.
Electron microscopy techniques used in this thesis were valuable tools to:
– characterize the particle size distribution (PSD) of the studied Pt/C electrocatalysts and follow their degradation mechanisms in identical regions (TEM and IL-TEM);
– follow the structural changes of the MEAs electrodes and Nafion® membrane during PEMFC stack operation (FEG-SEM);
The reader is referred to Ref. [5] for more details on the use of electron microscopy for characterization of PEMFC electrode materials.
Transmission Electron Microscopy (TEM)
A TEM is a microscopy technique where an incident beam of high energy electrons is transmitted through a sample. For the electron beam to penetrate the sample, it is required that the thickness of the sample is in the order of a few hundred nanometres. The electron beam is under vacuum and the presence of electromagnetic lenses allows the operator to control the convergence of the beam and the magnification of the microscope. The interaction of the electron beam with the sample forms a projection, allowing the operator to select the interest zones for imaging and the image results from either a mass/thickness contrast or a diffraction contrast. As for any experimental technique, the operator sensibility is extremely important to control the image properties (brightness and/or contrast) in order to obtain comparable images, once the interaction of the electron beam with the sample may provide measure artefacts [6].
For TEM experiments, the electrocatalysts were deposited onto a copper or a gold grid baring a Lacey carbon membrane (300 mesh, Lacey Carbon; Agar Sc. UK) and examined with a Jeol 2010 TEM operated at 200 kV with a point to point resolution of 0.19 nm located at the Centre des Moyens Technologiques Communs (CMTC) of Grenoble INP. The images were used to build the PSD of the electrocatalysts and to follow the structural changes of the Pt/C electrocatalysts before/after degradation test. From these observations, the number-averaged mean-particle size: were determined by eye-counting over ca. 400 particles (ni stands for the number of particles having a diameter di). Only “isolated” particles (that is non-agglomerated, single grain spherical particles) were counted to build the particle size distribution. Any other Pt nanoparticle whatever its shape or its structure (a single crystallite or a combination of individual nanocrystallites) was considered an “agglomerated” Pt nanoparticle.
Identical-Location TEM (IL-TEM)
The degradation mechanisms of PEMFC electrocatalysts were also determined using Identical-location Transmission Electron Microscopy (IL-TEM) [7-9]. In this technique, first introduced by Mayrhofer and co-workers, a TEM grid made of Au is loaded with a given electrocatalyst, used as a working electrode in a conventional electrochemical cell, and finally, the “same exactly locations” that were previously observed are imaged again in TEM.
Technically speaking, the electrocatalyst suspension was diluted by a factor of 4 with MQ-grade water (Millipore). A 5 μL aliquot was then deposited over a gold TEM grid (300 mesh, Lacey Carbon; Agar Sc. UK), and dried in air. Five randomly selected carbon particles located at different places of the grid were imaged at low (80 k) and high (150 k) magnification. Then, the grid was carefully clamped between two carbon plates, which served as a mechanical support and current collectors for the electrochemistry experiments. After the electrochemical experiments, the TEM grid was rinsed with MQ-grade water, dried in air, and TEM images were acquired in exactly the same position (identical-location TEM). The experimental steps of such technique are exemplified in Figure II – 4. The IL-TEM images were used to build the particle size distribution of the electrocatalysts before/after degradation test and to determine the number-averaged mean-particle size ( d N ) and the surface-averaged mean particle size ( d S ).
Figure II – 4. Sequence for IL-TEM experiments: 1) Deposition of the electrocatalyst suspension and TEM observation, 2 and 3) Clamping TEM grid between carbon plates 4) Fixation of the carbon plates with working-electrode contact 5) Electrochemical tests 6) Disassembling of the carbon plates 7) Observation of the same locations as in step 1.
Field-Emission Gun-Scanning Electron Microscopy (FEG-SEM)
In SEM, a finely focused electron beam is scanned over a conducting sample with the help of deflector coils. The interaction of the focused beam with the surface of the sample creates different signals among of which are: (i) secondary electrons (SE) (ii) backscattered electrons (BSE) (iii) characteristic X-rays and (iv) fluorescence X-rays. The signals used to form images are the secondary electrons and the backscattered electrons. Secondary electrons are sensitive to the topographical contrast given by the shape and morphology of the surface of the sample; while backscattered electrons provide compositional contrast due to the difference in the intensity of the signal with the atomic number of the scanned area (regions of high atomic number will be brighter relative to regions of low atomic number).
FEG-SEM images were obtained on a Zeiss Ultra 55 microscope designed to maximize the image resolution at low beam energies. In contrast to the V-shaped filaments made of tungsten (W) or lanthanum hexaboride (LaB6), the FEG allows the production of an electron beam that is smaller in diameter resulting in an improved signal-to-noise ratio and spatial resolution. The Ultra 55 is equipped with a secondary electron detector inside the lens (in-lens detector) and a backscattered electron detector. Both are engineered to image electrons at low accelerating voltage (less than 5 kV). The samples consisted of circular or square zones cut fresh/aged active layers of MEAs. The measurement of the component thickness was done by integrating the overall thickness of the electrodes or membrane with ImageJ® freeware.
Chemical techniques
Raman spectroscopy
Raman spectroscopy is a non-destructive technique used to identify the chemical composition, characterize the molecular structure and quantify the elements present in a material. Raman spectroscopy is a very versatile technique: it requires minimal sample preparation, allows the characterization of materials in different phases (solid, liquid or gases), pure chemical elements or solutions, and is used in several fields of research from pharmaceutics [10], medicine [11] and characterization of carbon materials [12-14], among others.
During Raman spectroscopy measurements, the sample is irradiated with monochromatic light, which produces reflected, adsorbed and scattered radiation (Raman scatter) of different wavelengths. The scattered phonons provide information on the vibrational modes, chemical and structural nature of the material. A Raman spectrum is a plot of the intensity of Raman scattered radiation as a function of its frequency difference from the incident radiation (usually in units of wavenumbers, cm-1), featuring characteristic peaks that are specific of molecular bonds.
Raman spectroscopy was used to examine the structure of the fresh/aged carbon supports. The Raman spectra were recorded ex situ using either a Renishaw RM1000 or a Renishaw In-Via spectrometer. The Raman spectra were obtained by excitation with the radiation from an argon LASER (514 nm) operated at approximately 5 mW. The detector was a Peltier-cooled charge coupled device camera (CCD) and the spectral resolution was about 1 cm-1. The measurements were performed with a x 50 ULWD objective and a 100 µm confocal aperture for both the sample illumination and collection of the scattered photons. For the sake of comparison between the samples, the Raman spectra of the Pt/C electrocatalysts were normalized by the intensity of the peak at ca. 1600 cm-1, which corresponds to the band of the graphitic lattice. Curve fitting for the determination of spectral parameters was performed with the software LabSpec. An accurate fitting of the Raman spectra considers the presence of 5 bands (Table II – 1).
Table of contents :
Chapter I. Introduction
1.1 Fuel cell technologies and industrial overview
1.1.1 Energy issues of today’s society
1.1.2 Fuel cell technology: operating principle, applications and types of fuel cells
1.1.3 Fuel cell industrialization and market
1.2 PEMFC – Proton Exchange Membrane Fuel Cell
1.2.1 Single cell: electrochemical reactions and membrane electrode assembly (MEA)
1.2.2 Proton Exchange Membrane (PEM)
1.2.3 Catalytic layers
1.2.4 Gas diffusion layers (GDLs)
1.3 From single cell to stack and fuel cell system
1.4 Degradation mechanisms of MEAs during PEMFC operation
1.4.1 PEM and ionomer degradation during PEMFC operation
1.4.2 Pt nanoparticles degradation during PEMFC operation
1.4.3 Carbon support degradation during PEMFC operation
References
Chapter II. Experimental Section
2.1 Electrocatalysts
2.2 Electrochemical measurements
2.3 Preparation of the catalytic inks and of the porous catalytic layers
2.3.1. Preparation of the catalytic inks
2.3.2. Preparation of the porous catalytic layers
2.4 Electrochemical characterization
2.5 Accelerated stress tests (ASTs)
2.6 PEMFC testing
2.7 Ex situ preparation of the fresh/aged MEAs cathode electrocatalysts
2.8 Characterization of the fresh and aged electrocatalysts
2.8.1 Physical techniques…
2.8.2 Chemical techniques
2.8.3 Electrochemical techniques
References
Chapter III. From Pt/C electrocatalysts suspensions to accelerated stress tests
3.1 Introductory note to Raman spectroscopy of carbon materials and accelerated stress tests protocols
3.2 Structural properties of carbon supports
3.3 Preparation of catalysts’ suspensions
3.3.1 Definition of the initial surface state and effect of Nafion® on the carbon support
3.4 Effect of storage of the ink under air atmosphere on the carbon support
3.4.1 Effect of short-term storage of the ink
3.4.2 Effect of long-term storage of the ink
3.5 Role of intermediate characterizations, nature of the carbon support, gas atmosphere and potential limits on ECSA and Q/HQ redox couple during ASTs
3.5.1 Electrochemical evidences on the influence of intermediate characterizations on ECSA and Q/HQ redox couple evolution
3.5.2 Structural modifications of the Pt/HSAC electrocatalyst induced by intermediate characterizations followed by IL-TEM
3.6 Influence of the nature of the carbon support, the gas atmosphere and the potential limits used in ASTs
3.6.1 Electrochemical results
3.6.2 Structural modifications of the carbon supports after the ASTs
3.6.3 Structural modifications of the Pt nanoparticles after the ASTs
3.7 Conclusion
References
Chapter IV. Carbon corrosion in PEMFCs : from model experiments to real-life operation in MEAs
4.1 Introduction
4.2 Results and Discussion
4.2.1 Electrochemical characterization
4.2.2 Physical characterization
4.2.3 Chemical characterization
4.2.4 Comparison with MEA operated on site during 12,860h
4.2.5 Degradation mechanism of HSAC supports
4.3 Conclusion
References
Chapter V. Carbon corrosion induce by membrane failure: the weak link of PEMFC longterm performance
5.1 Introduction
5.2 Results and Discussion
5.2.1 110 cell system operating performance
5.2.2 In situ evidences of performance heterogeneities at the stack and the MEA level
5.2.3 Linking degradation of electrical performance to membrane failures
5.2.4 Linking the degradation of the cathode catalytic layer and of the electrical performance to the membrane failures
5.3 Discussion
5.4 Conclusion
References
Chapter VI. Conclusion and Outlook
