Quantitative in Situ Analysis of Ionomer Structure in Fuel Cell Catalytic

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Quantitative in-situ analysis of ionomer structure in fuel cell catalytic layers Tobias Morawietz, Michael Handl, Claudio Oldani, K. Andreas Friedrich, and Renate Hiesgen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07188 • Publication Date (Web): 23 Sep 2016 Downloaded from http://pubs.acs.org on September 25, 2016

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ACS Applied Materials & Interfaces

Quantitative In-Situ Analysis of Ionomer Structure in Fuel Cell Catalytic Layers Tobias Morawietz1, Michael Handl1, Claudio Oldani2, K. Andreas Friedrich3, and Renate Hiesgen1* 1

University of Applied Sciences Esslingen, Kanalstrasse 33, 73728 Esslingen, Germany

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Solvay Specialty Polymers, R&D Center, Viale Lombardia 20, 20021 Bollate (MI), Italy

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German Aerospace Center, Institute of Engineering Thermodynamics, Pfaffenwaldring 38-40,

70569 Stuttgart, Germany

ABSTRACT: A quantitative in-situ investigation of the structure of the catalytic layer of polymer electrolyte membrane (PEM) fuel cells using material-sensitive and conductive atomic force microscopy (AFM) is reported. The distribution and size of the ionomer phase at the surface of the catalytic layer is retrieved from adhesion force mappings, measured at high humidity and up to 75°C. The average ionomer layer thickness varies between 7 to 13 nm for three differently prepared samples, as concluded from the histograms. Evidence of a lamellar structure of the thinner ionomer layers is presented. A significant thinning of the ionomer layers after long-term fuel cell operation is observed. .

KEYWORDS: AFM, fuel cells, electrodes, ionomer layers, degradation INTRODUCTION In the end of 2015, an international worldwide agreement resulting from the COP 21 climate conference in Paris again emphasized the importance of CO2-free energy technologies. Fuel cells fed with renewable hydrogen provide highly efficient, carbon-free energy conversion from chemical energy into electricity and heat, and they could contribute significantly toward reaching the climate goals. In particular, low-temperature polymer-electrolyte membrane (PEM) fuel cells have shown promise in electromobility applications, and they have gained renewed interest because hydrogen-fueled cars offer the possibility of fast refueling times with acceptable driving ranges; the first commercial cars have recently been introduced into the market. Major advances in the performance and durability of fuel cells are needed. According to recent studies, the cost of precious metals will remain important even for mass production.1 Still, Pt is primarily used as the catalyst of electrochemical reactions in the electrodes, and a significant amount of research and development focuses on reducing the platinum loading. Different strategies for Pt reduction

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are being followed; the most important include alloys with other metals, core-shell concepts, and thin layers of Pt, Pt-free carbon catalysts, and even anion-exchange membranes. However, all of these developments have not reached the technical maturity required for commercial fuel cell applications.2 3 4 5 6 In this work, PEM fuel cell electrodes with reduced platinum loading were investigated. The membrane and electrodes constitute the main parts of the fuel cell or MEA. Porous electrodes are a mixture of mesoporous carbon (15-30 nm), supported platinum catalyst particles 1-4 nm in size, and 30-40% of an ionically conductive polymer (tetrafluoroethylene-perfluoro sulfonic acid vynilether copolymer), such as Nafion® (Chemours, Wilmington, USA) or Aquivion® (PFSA) (Solvay Specialty Polymers, Milan, Italy), as the electrolyte and binder. The performance of the fuel cell and the best utilization of platinum depend on the optimal transport of protons, electrons, and gases through the electrodes. However, the structure and size of the Pt/C-ionomerparticles/agglomerates inside the electrode are still unknown. Knowledge of the detailed structure is valuable for accurate modeling of the MEA.7 The thickness of the ionomer layers that cover the catalyst particles/agglomerates has been shown to have a significant influence on the performance. In the cathode the ionomer layer thickness determines the mass transport resistance of oxygen to the reaction sites and thereby determines the fuel cell performance.8

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study of Kongkanand et al. strongly emphasized the penalty of the local oxygen resistance of the ionomer in the cathode in case of low-platinum loaded fuel cell electrodes for reaching a high performance.11 In addition to the diffusion coefficient also the thickness of the ionomer that covers the catalyst is of importance. Therefore, the detailed structure of the electrodes is crucial and has been intensively investigated.12 13

Because the components of the electrodes all have structures with dimensions on the nanometer scale, only analysis techniques with sufficiently high lateral resolutions are suited for their investigation. In the past, electron beam-based analysis techniques, such as transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray analysis (XRD) and combined techniques, such as scanning transmission electron microscopy (STEM), have mainly been applied to analyze the structure of electrodes in detail. Recent tomographical approaches comprise synchrotron-based soft X-ray scanning transmission microscopy (STXM) and highangle annular dark-ﬁeld scanning transmission electron microscopy (HAADF-STEM). 141 15 16 13


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Notably, electron beam-based techniques induce radiation damage in the polymer, thus altering the sample; however, this influence has been significantly reduced in advanced modern techniques such as HAADF. Furthermore, they are performed under vacuum, causing drying and shrinking of the polymer content with an unknown change in the dimensions. From TEM measurements, More et al. derived thicknesses of the ionomer layers around the catalyst agglomerates ranging from 5-10 nm.17 18 In general, the determination of the ionomer content or the thickness of the layers from TEM or SEM has proven difficult due to their low contrast with carbon. An HAADF study of ionomer layers has recently been reported by Lopez et al.16 They analyzed material that was scratched out from platinum-free model electrodes and prepared from Cs-ion-exchanged ionomer-covered carbon to enhance the contrast. An average layer thickness of approximately 7 nm measured in vacuum was reported and was independent of the preparation procedure but having different ionomer coverage. Other approaches used modeling of the electrode structure based on experimental input data to report calculated mean layer thicknesses ranging from 4 to 15 nm depending on ionomer content in the range of 14% to 50%, respectively.19 Still, the structure and thickness of the thin ionomer layers around the Pt/C particles at close to operational conditions have not been clarified. In model experiments, selfassembled ultra-thin Nafion® layers, which were deposited on different substrates, resulted in a minimum layer thickness of 4 nm.20 21 22 Modeling of the structure of a thin Nafion® ionomer on a substrate, in the case of a hydrophilic surface, leads to a sandwiched polymer-covered water layer, and on a hydrophobic surface, it leads to a polymer bilayer with an enclosed water film. Both scenarios exhibit total thicknesses of approximately 4.5 nm.23 After operation, a main factor in the degradation of the electrodes is the loss of the electrochemically active surface area (ECSA). The loss of ECSA is thought to be caused primarily by the dissolution of Pt, the migration and coalescence of Pt nanoparticles on the support, and the detachment of nanoparticles from the support; the latter is mainly connected to carbon oxidation.24

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The nanoscopic thinning of the ionomer layers that cover the Pt/C

particles inside the electrodes under close-to-operational conditions has not been reported. Atomic force microscopy (AFM) is a high-resolution surface analysis tool. It has been proven to deliver deeper insight into the structure and conductivity of Nafion® and Aquivion® PFSA ionomer layers, membranes, and electrodes.22

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Using material-sensitive and conductive


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AFM, different materials can be identified by their properties, which are retrieved as mappings correlating to the topographic data.32 In the electrode cross-sections, the ionomer layer can be distinguished from carbon and platinum by its higher adhesion force and stiffness (DMT modulus) as well as its lack of electronic conductivity.31 In this study, we report on the structure of electrodes based on Nafion® and Aquivion® PFSA before and after fuel cell operation. We determine the ionomer layer thicknesses in the electrodes in near operational conditions. The cells were operated in hydrogen/air mode in a single cell with 25 cm2 at 80°C and a relative humidity (RH) of 50% or 80%. The AFM measurements were performed at 60% RH, at room temperature and up to 75°C (details on MEAs are given in experimental section).

EXPERIMENTAL SECTION MEA Operation. Commercially available MEAs purchased from Johnson Matthey (Johnson Matthey Fuel Cells, Swindon, UK) with an anode loading of 0.2 mg/cm² Pt and a cathode loading of 0.4 mg/cm² Pt (“commercial MEA”) having a reinforced Nafion® XL membrane of with a nominal thickness of 27.5 µm and Nafion® as binder in the electrodes were used. The other two MEAs were prepared in the laboratory with different preparation methods (not disclosed). “Experimental MEA I” was prepared with a reinforced Aquivion® PFSA membrane having a 20 µm thickness and equivalent weight of 790 g/mol and commercial Aquivion® PFSA D79-20BS as the binder in the electrodes, and had anode and cathode loadings of 0.2 mg/cm² Pt. ”Experimental MEA II” had a reinforced Aquivion® PFSA membrane with a nominal thickness of 10 µm and equivalent weight of 790 g/mol and had an anode loading of 0.05 mg/cm² Pt and a cathode loading of 0.2 mg/cm² Pt; Aquivion® PFSA dispersion D79-20BS was used as a binder in electrodes, too. Aquivion® PFSA materials are available from Solvay Specialty Polymers, Bollate (Milan), Italy. The MEAs were operated at DLR Institute of Engineering Thermodynamics, Stuttgart, in a single cell setup with an area of 25 cm2 for 235 h at 80°Ca current load of 1 A·cm-2, and a pressure of 1.5 bar. The commercial MEA was operated at a relative humidity (RH) of 50%, the experimental MEA I at RH 80%. After operation, these samples were cut out of the MEAs near the hydrogen inlet.


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Scheme 1. AFM measurement of a MEA cross-section sample.

Sample Preparation. The samples were cut of the operated MEAs, from the center in case of experimental MEA II, and close to the hydrogen inlet in case of commercial MEA and experimental MEA II. For the cross-sectional analysis, the MEA samples were embedded in Terosion Teromix PU6700 (high viscous polyurethane, BASF, Ludwigshafen, Germany) and cured for at least 24 h at room temperature. For the AFM analysis, the samples were cut using a microtome (Leitz, Jena, Germany) and fixed using double-sided adhesive conductive tape on 12 mm-wide metallic AFM sample discs (Plano, Wetzlar, Germany) (Scheme 1).

AFM Operation. A Bruker Multimode 8 AFM (Karlsruhe, Germany) was used for materialsensitive and conductive AFM analyses. The samples were measured in quantitative nanomechanical tapping mode (QNM™, Bruker Corp., USA). Simultaneously with topography data recording, mappings of the adhesion force and stiffness (DMT modulus) were extracted. For high-resolution measurements, AFM probes with a diamond-like carbon (DLC) tip and a nominal radius of less than 1 nm were used (SHR150, Nanosensors). For controlled humidity, the AFM head was placed in a gas-tight chamber equipped with a humidity control unit (Cigar Oasis Ultra, Cigar Oasis, USA). Using a conductive AFM tip (PtIr coated tips (PPP-NCHPt, 42 N/m; Nanosensors, Neuchatel, Switzerland) and an applied voltage between the AFM probe (at ground) and the sample holder, the current signal in tapping mode was recorded simultaneously with the mechanical properties and the topography using a current-voltage amplifier, and they were averaged using a lock-in amplifier (PF-TUNA Module, Bruker, USA). The conductive areas were determined by the sensitivity of the amplifier of approximately 1 fA. If current flow was detected, this area was considered to be conductive. Unless otherwise stated, averaging was


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performed over the tip-sample contact time. For analysis, images 1 µm x 1 µm in size were recorded at a scan speed of 0.488 Hz with 1024 Pixels/µm.

Histogram Evaluation. The analysis of the ionomer layer thickness was performed by evaluating the adhesion mappings. As measured for the ionomer layer thickness, the distance was taken between the two turning points of a line section drawn perpendicular to the layer. All layers in the image were evaluated, and their relative frequencies were plotted as a histogram against the thickness, with a bin size of 0.5 nm. The peak position derived from a log-normal fitting of the data was used for the measurement of the average ionomer layer thickness. The resulting uncertainty of the peak position was estimated as ∆d= ± 0.5 nm. The shift in fitted peak positions from the histograms, which were evaluated from two different images and recorded at neighboring sample spots, was determined as 0.3 nm.

Results and Discussion 2.1 Structure of Electrode Cross-Section. In Figure 1, an AFM topography image (Figure 1a) of the cross-section of the experimental MEA I and the same topography image overlaid with the corresponding current mapping (Figure 1b) with a side length of 30 µm is shown. At the top and bottom, the two similar, electronically conductive electrodes are visible. Even on this large scale, a significant heterogeneity of the conductivity can be observed. The ionomer layers that sandwich the reinforcement are swollen in comparison with the reinforcement layer in the center. No ionic conductivity is observed in the membrane at U= 100 mV.31


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. Figure 1. a) AFM 3D-topography of a cross-section of the Aquivion® PFSA-based experimental MEA I. b) AFM 3D-topography of a) overlaid with current recorded at U= 100 mV, images measured at RT and 30-40% RH.

At a higher resolution, the rough porous electrode of the Nafion®-based commercial MEA is visible in the topography image of Figure 2a with an area of 1 µm x 1 µm. In the corresponding adhesion mapping (Figure 2b), the three components of the electrode, the ionomer, carbon, and Pt-catalyst particles/agglomerates, can be discerned by the strength of their adhesion forces, with the highest adhesion referring to Nafion®. Individual platinum particles cannot be resolved on this scale, but Pt-rich surface areas on the carbon appear black due to their very low adhesion force; whereas, the Pt-free carbon surface appears brown.31 In the stiffness mapping in Figure 2c, the highest stiffness (bright) refers to platinum at or beneath the surface, the ionomer is soft and appears dark. For a 3-D view onto the electrode (Figure 2d), a color-coded adhesion map has been overlaid onto the topography to enhance the position of the platinum on the carbon particles and the ionomer-covered areas. The approximate range of the adhesion force for the different materials is written on the color-coding bar. In the image, Pt is identified by the red areas on the yellow/green carbon agglomerates that are embedded in the ionomer, colored in purple. In Figure 2d, the internal structure of the platinum-rich carbon particles, which are partly colored in red, is visible from the adhesion and stiffness mappings that indicate the platinum coverage. Their dimensions, which are roughly 100 nm, indicate that they are agglomerates. The pores inside the electrode are difficult to recognize in the AFM image. Pores may be located at


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the deep depressions that are visible in the topography image (Figure 2a), but they have not been quantified. The agglomerate sizes of the anode and cathode of all three samples have been evaluated from the adhesion images, and they range between 70-100 nm.

Figure 2. AFM images of a cross-section of the cathode of the Nafion®-based commercial MEA, measured at RT and 50-60% RH. a) The topography image. b) The adhesion image, in which brighter colors indicate higher adhesion and refer to the ionomer; black/brown refers to the platinum and carbon, respectively. c) The stiffness (DMT modulus) mapping. d) A 3dimensional view of the electrode surface overlaid with the adhesion mapping.

2.2 Ionomer Area. A determination of the catalyst relative ionomer coverage of the imaged area has been performed in two ways for all three different MEAs and electrodes at a relative humidity of 50-60%: (1) the analysis of the percentage of the high adhesion surface area coverage, and (2) the determination of the percentage of the non-conductive surface area


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coverage. The non-conductive area was determined from the simultaneously recorded current image by subtraction of the value of conductive area from the total area. Its value should correlate with the ionomer covered area and serves as an additional confirmation. In Figure 3a, all average values are given together with the mean error of the average for a confidence level of 68%. In Figure 3b, the non-conductive area of 42% is marked in red, and the conductive area is marked in blue. In Figure 3c, an ionomer area of 47%, determined from the high adhesion surface area of the anode of the commercial Nafion®-based MEA, is marked in green. Each value was determined from three different 3 µm x 3 µm AFM images while simultaneously recording the adhesion force and current. Within the uncertainty range, a coverage of approximately 50%, which was determined from both the non-conductive surface and the high adhesion area, is observed for all samples. In the literature differing values for the optimum Nafion content are reported. Sasikumar et al. have measured an increase of optimum Nafion content with decrease in Pt loading.33 Using a 20% Pt/C catalyst with 0.5, 0.25 and 0.1 mg/cm2, best performance was obtained at about 20, 40, and 50 wt% Nafion loading, respectively. Passalacqua et al. report own measurements and further data from the literature, which differ from the above results.34 Using catalysts with 10-25% Pt/C and a loading ranging from 0.5 to 1 mg/cm2, an optimum Nafion content of 30-40 wt% was assumed. A recent publication of Yu et al. reports more literature data with similar results.35 Taking into account the low Pt loading of the electrodes used in this work, an ionomer content of at least 40 wt% is expected. Assuming a homogeneous distribution, for an ionomer content of 40 wt% an ionomer area coverage of 33% results. The experimental area coverage of 50% is higher, but may be reasonable taking into account the non-uniform ionomer distribution. The encasement of the Pt/C agglomerates with a thin ionomer layer leads to a higher area coverage. In addition, the ionomer area strongly depends on humidity. With a decrease of RH from 60% to 30%, the ionomer content decreased to 27%, as measured on the same area. At an RH of 60%, an ionomer coverage larger than the nominal value would therefore be expected.


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Figure 3. a) The ionomer coverage of the different MEAs measured at RT and 50-60% RH. b) The current mapping of the anode of the Nafion®-based commercial MEA; the current is marked in blue and the non-conductive area is marked in red (42%). c) The adhesion mapping of the same area as b) with an ionomer coverage of 47%, marked in green.

2.3 Agglomerates. An evaluation of the average size of Pt/C agglomerates, retrieved from the 3 µm x 3 µm AFM adhesion images, including all 5 different electrodes (MEA I has the same Pt loading on the cathode and on the anode) from the three MEAs, revealed a linear dependence in the agglomerate size versus Pt loading (Figure 4). In Figure 2c and in Figure 5, Pt/C agglomerates are visible in the anode of the commercial MEA. Their diameters ranged between 70 and 100 nm, which is in agreement with reports from the literature. An average size of 100 nm was modeled by Moore.36 Through SEM analysis, Owejan found an agglomerate size of approximately 70 nm, which was independent from Pt loading.37 Due to the vacuum conditions required for SEM measurements, the agglomerate size under humid conditions is most likely larger.


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Figure 4. The dependence of the mean Pt/C agglomerate size of the electrodes of three different MEAs, retrieved from adhesion mappings, as measured at 50-60% RH and RT.

A closer view of the zoomed-in images comprising only a few agglomerates from the anode of the commercial MEA is given in Figure 5. The agglomerates are embedded in the brightly colored high-adhesive ionomer (Figure 5a). The evaluation of the ionomer coverage of individual agglomerates is shown in Figure 5b, in which the agglomerate area visible in Figure 5a has been overlaid with the simultaneously recorded conductivity mapping. A comparison of the agglomerate coverage evaluated over larger areas is given in Figure 5c. On average, a coverage of approximately 40-50% was found despite the different materials used and preparation methods, which is in agreement with data reported by Ikeda.38 Using 3-D HAADFSTEM analysis, Lopez et al. reported an ionomer coverage of 80 and 40% for agglomerates from differently prepared model electrodes with high and low ionomer content, respectively; although, these values were without platinum loading.16 It has to be taken into account that the coverage determined from the ionomer surface area, as performed in this study or by 3-D tomography, could be lower than the coverage that was present before cleavage or scratching.

Figure 5. a) Adhesion mapping of the anode of the commercial MEA. b) The mapping of the conductive areas overlaid on the areas of agglomerates. c) A comparison of the ionomer coverage of agglomerates for all electrodes.

High-resolution measurements of the commercial MEA anode indicate separated ionomer layers around different agglomerates in the electrode. In Figure 6a, the high-adhesion ionomer area, measured at 30-40% RH, is marked in blue, with a few positions of clearly distinguishable


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ionomer layers marked with arrows. In Figure 6b, a high-resolution adhesion mapping is shown, in which carbon agglomerates are joined by a bright ~14 nm-thick ionomer layer, which is composed, at 25% RH, of two separated layers (Figure 6c). For the electrode structure, two models have been proposed: the embedded structure where Pt/C is embedded into the ionomer film as matrix, and the encapsulated structure. The encasement of Pt/C agglomerates in separate ionomer layers has been proposed in the literature derived by modeling, but has not yet been directly observed under humid conditions.13 The results strongly favor the model of encapsulated agglomerates since individual films on separated agglomerates are observed.

Figure 6. a) A high-resolution adhesion mapping of the commercial MEA anode measured at RT and 30-40% RH; the ionomer is marked in blue, and the arrows mark the positions of the separated layers. b) The zoomed-in image of a high-resolution adhesion mapping of the commercial MEA anode with two distinguishable layers around the Pt/C agglomerates measured at RT and 25% RH. c) A cartoon of ionomer-enclosed Pt/C agglomerates.

2.3 Ionomer Layer Thickness before Operation. To determine the distribution of the thicknesses of the ionomer layers between the carbon particles/agglomerates in the electrodes, adhesion images of 1 µm x 1 µm were evaluated. An ionomer layer between two carbon particles/agglomerates was identified by its higher adhesion and non-conductivity. The directions of the adhesion force and current values, drawn perpendicular across the space between two carbon particles (the adhesion and current mapping in Figures 7a and b, respectively) exhibit their inverse behaviors. As a measure, the turning points of the adhesion peak, which match the crossing of the two signals, were determined as the layer thickness (Figure 7c). The histograms of all ionomer layer thickness values of an electrode were measured at MEA cross-sections at 50-60% RH and room temperature (RT), unless otherwise stated. The distribution of the ionomer layers from approximately 300 values in one image in the cathode of a commercial Nafion®-


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based MEA before operation is shown in Figure 8. A wide, asymmetric distribution is found for the ionomer layer thickness values (the blue bars in the foreground), ranging from 4 to 17 nm. As a measure for the average thickness, the position of the peak at a thickness of 8 nm, derived from a log-normal fit of the data (the dashed lines in Figure 8), was taken. The second histogram (the red bars in the background) was evaluated at a slightly different sample position. The close similarity of the two distributions confirms the reproducibility of the measurement and data evaluation.

Figure 7. AFM images of a cross-section of the cathode of the commercial MEA, as measured at RT and 30-40% RH and U=50 mV. a) The adhesion force: Pt-rich area appears dark. b) The corresponding current image with high current (bright) at Pt-rich areas. c) The adhesion force (black) and current profile (blue) measured across the ionomer layer between two particles along the marked lines in a) and b), respectively.


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Figure 8. A histogram, together with the fitted log-normal curves as the dashed lines, of all ionomer thickness values from two different positions covering an area of 1 µm x 1 µm, measured from the cross-sections of a Nafion® -based commercial MEA cathode at RT and 5060% RH.

To determine the minimal layer thickness, the thickness data measured at the positions without a height difference between two carbon particles were selected. The smallest ionomer layer thickness before operation was determined as 4-5 nm, which is a value that fits to the results from modeling the structure of Nafion® films on substrates, as reported by Borges et al., and from the results by Paul et al. of the deposition of ultra-thin Nafion® layers, which showed a minimal layer thickness of 4 nm for self-assembling layers on different substrates.23

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authors suggested a lamellar structure for these layers with roughly a 4.5 nm thickness. Depending on the substrate surface energy, Borges reported a water film at the interface or enclosed into the PTFE backbone strands; Paul et al. discovered a hydrophilic surface for selfassembled layers on different substrates below a thickness of approximately 10 nm. According to Ohira, who performed local AFM conductivity measurements, the conductivity perpendicular to the layer vanishes below a film thickness of 10 nm.40 Borges et al. reported that the formation of micelles started with an increasing thickness of the films and may hinder the transport of water/current, leading to a higher resistance. The modeling experiments of Borges et al. also implied a water film at the interface between a hydrophilic surface, such as Pt or hydrophilic carbon and the Nafion® backbone strands, which would allow a current to flow along the Pt/C particle surfaces. At a hydrophobic surface, which might also be present as part of the carbon surface, the water film is partly enclosed between two PTFE-like backbone layers, and due to the formation of micelle-like structures, may no longer be continuous.39 In conclusion, it can be


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assumed that the ionomer layers inside the investigated electrode having a thickness of smaller than 10 nm are mostly lamellar. In contrast to the modeling and set-up of the ultra-thin layers in the fuel cell electrodes, the Nafion® film is enclosed by Pt/C on both sides. As a first approach for two hydrophilic neighboring surfaces, one can assume that a water film is present on both sides of the backbone PTFE strands with a slightly higher layer thickness, in accordance with the minimal layer thickness measured with AFM. With mixed hydrophilic/hydrophobic surfaces, the ionomer structures likely increase in complexity, including the introduction of lamellar layers and micelles, and correspondingly the resistance to ionic current flow likely increases as well. The uniform coverage of the Pt/C particles with an ionomer layer of less than 10 nm, as present in the investigated electrode of the commercial MEA with 8 nm on average, seems to be advantageous for providing a small diffusion resistance to gases and a high ionic conductivity. The change of the ionomer layer thickness, which is induced from increasing the temperature from 25°C up to 75°C by heating the sample from the bottom using a piezo heater and subsequently cooling down to the starting temperature of 25°C, is shown in Figure 9. The equilibration time between subsequent temperature steps was 1 h, and the temperature and relative humidity of the surrounding atmosphere remained constant. Upon heating the sample, the air above the sample likely warmed up, and the relative humidity above the electrode and at the surface may have changed, depending on the velocity of the water transport from the bulk to the sample surface. These conditions also likely occur in fuel cells under operation. At a temperature close to fuel cell operation temperature (75°C), the average layer thickness, measured as the peak position of a log-normal fit, almost doubled. After cooling the sample back down to 25 °C, the layer thickness remained permanently larger by approx. 1.5 nm (25% of the initial value). A similar thickness hysteresis was also observed by Kalisvaart et al. using X-ray reflectivity analysis of 15 nm-thick Nafion® films deposited on SiO2.41 This hysteresis is a consequence of the visco-elastic properties of the polymer. Following the extension forced by the water uptake and removal, a residual irreversible extension remains.32 The significant thickness increase induced from a change in temperature leads to tensile stress in the electrodes which may accelerate degradation. In addition, the irreversible thickness changes sum over time and are most likely associated with permanent structural changes in the electrode, which may unfavorably influence the performance.


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Figure 9. The dependence of the ionomer layer thickness of the commercial MEA cathode on heating (red curve) and subsequent cooling (blue curve), as measured at 30-40% RH.

The ionomer layer distributions of the three different electrodes with different (not disclosed) preparation methods and two different ionomers are compared in Figure 10. They differ significantly in their average ionomer layer thickness (peak position) depending on the preparation method. The experimental MEA I has an average ionomer layer thickness of 12.7 nm, and the distribution spreads from 8 to 20 nm; the commercial electrode has an average thickness of 8.5 nm, and the thickness spans from 5 to 14 nm. The peak positions derived by a fit to the histogram represent the layer thickness with the highest probability. They agree well with the average ionomer layer thickness range derived by modeling, reported by Suzuki et al.19 The thinnest average ionomer layer belongs to the experimental MEA II, with a thickness of 7.0 nm and a range from 4 to 12 nm. The broad distribution of experimental MEA I indicates a second peak with a layer thickness roughly double the size of the first layer. Indeed, high-resolution measurements of the pristine commercial MEA cathode revealed the existence of two separated layers between two carbon particles/agglomerates (Figure 6b).


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12

Relative frequency / %

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Experimental MEA II Commercial MEA Experimental MEA I

10 8 6 4 2 0 0

2

4

6

8

10 12 14 16 18 20 22 24

Ionomer layer / nm Figure 10. The relative frequency of the ionomer layer thickness of the experimental MEA II (red bars) anodes with Aquivion® PFSA as the ionomer, the commercial Nafion®-based MEA (green bars), and the experimental Aquivion® PFSA-based MEA I (blue bars). All measurements were performed before operation at RT and 50-60% RH on the cross-sections.

For the commercial MEA and the experimental MEA II, with an average ionomer layer thickness of 7-8 nm, a lower oxygen resistance was present. In addition, the structure of the ionomer layers in the electrodes are considered to be mostly lamellar, if one assumes two ionomer bilayers sandwiched between the neighboring Pt/C agglomerates. A lamellar ionomer would provide no conductivity across the ionomer to bridge two Pt/C particles, but a current would flow along the Pt/C surface. The formation of micelles with a more complicated water/current path that induces a higher resistance is more likely only for the thicker ionomer films, which account for roughly one-third of the total layers, and most likely in the case of the experimental MEA I with an average layer thickness of 13 nm. The electrodes with the smallest and largest average ionomer layer thicknesses were both based on Aquivion® PFSA. Although the influence of the preparation method on ionomer thickness cannot be revealed in detail here, the great influence of preparation on the average thickness is obvious and corresponds with reports in the literature.42 43 44 45 13 38 46 47 40

2.4 Influence of Fuel Cell Operation on Ionomer Layer Thickness. After the fuel cell operated for 235 h, for the commercial MEA and the experimental MEA I the thickness of the ionomer layers in both electrodes was evaluated again. A comparison of the relative occurrence


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of ionomer layer thickness values measured at the new and the operated anode and cathode is given in Figure 11a, b for the commercial low degradation rate MEA and in Figure 11c, d for the experimental MEA I. Thinning of the ionomer layer thickness occurred for the samples. However, for the commercial MEA, no change was found at the cathode, and an average thickness decrease of approximately 3 nm of the anode-side layer thickness was observed. The ionomer layer thickness of the experimental MEA I decreased by 5 nm at the anode and 4 nm at the cathode. Measured thickness and the relative reduction after operation in fuel cell of ionomers in cathode and anode of the two analyzed MEAs are resumed in Table 1.

Table 1. Thickness of ionomer layers (cathode and anode) in pristine and operated (235 h) MEAs and relative thinning. Commercial MEA MEA I Cathode

Anode

Cathode Anode

7

9

12

12

Thickness operated /nm 7

6

8

7

Thinning / %

33

33

42

Thickness pristine /nm

0

Such significant thinning of the ionomer layers in the electrodes has not been reported before, and the contributing factors will be discussed in the following chapter. Comparing the performances of these 2 MEAs, the experimental MEA I had an initial voltage of 0.65 V at 1A/cm², with a degradation rate of ∆U= -0.95 mV/h. The commercial MEA had an initial voltage of 0.60 V at 1A/cm² and a degradation rate of ∆U= -0.11 mV/h. Comparing the performances of the MEAs, the MEA I with the thickest ionomer layers had a high degradation; an advantage was observed for the commercial MEA with a thin lamellar structure. Although, the fast degradation of the experimental MEA I might have had several reasons, a high oxygen diffusion resistance must have been present in the cathode as a result of the thick ionomer layers.


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Figure 11. The relative frequency of the ionomer layer thickness for the commercial MEA and the experimental MEA I (both samples from hydrogen inlet) before and after 235 h of operation, as measured at 50-60% RH at RT. a) The anode of the commercial MEA. b) The cathode of the commercial MEA. c) The anode of the experimental MEA I. d) The cathode of the experimental MEA I.

In both MEAs, the thinning of ionomer layer after operation at comparable conditions was greater at the anode (Table 1). The whole ionomer layer thickness distribution shifted to smaller values indicating an overall thinning trend for the ionomer layers (Figure 11). Both samples originated from a position close to the hydrogen inlet. An increased thinning of the membrane near the hydrogen inlet has been reported by de Moor et al. and was explained as an attack from radicals, including hydrogen radicals.48 From the experimental investigation of fuel cell membranes, it is known that greater ionomer degradation occurs under dry conditions.49

50

In

contrast, the degradation of platinum accelerates at higher humidity or flooding conditions, which typically occur at the cathode.26 51 During operation, the relative humidity is smaller at the anode than at the cathode, where the reaction water is formed. In the presence of oxygen gas and preferably at a potential around zero volts (at the anode), hydrogen peroxide is formed and leads to the formation of oxygen-based radicals such as hydroxyl radicals.52 Hydroxyl radicals can


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directly attack the ionomer and induce side-chain unzipping.53

54 55

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This reaction can occur at

the anode when oxygen gas has permeated through the membrane. In addition, Coms described a continuation of the ionomer degradation pathway, which is especially valid close to the anode.53 In the presence of hydroxyl radicals and hydrogen, highly active hydrogen radicals are formed and can also attack the PTFE chains. This mechanism would explain the increased ionomer thinning found on the anode side compared to the cathode electrode.48 At the cathode, the hydrogen concentration is much lower and the formation of hydrogen radicals is unlikely. With the use of thinner membranes, i.e., 10 µm of experimental MEA I instead of 28 µm, as in the case of the commercial MEA, the amount of oxygen that reaches the anode by permeating through the membrane is most likely increased. A thinner membrane would also increase the amount of permeated hydrogen at the cathode side, and promote the degradation of the ionomer. In agreement with these conclusions, only a negligible layer thinning was measured at the cathode side of the commercial MEA with a 28 µm-thick membrane, in contrast to the experimental MEA I, which had a thinner 10 µm-thick membrane. In addition, during operation, the gas permeation further increases if thinning of the membrane occurs, and increased gas permeation further accelerates the ionomer degradation. An inhomogeneous degradation resulting from H2O2/Fe2+ that has been reported from model experiments may also be taken into account.56 During operation, a decrease in the oxygen diffusion resistance in the cathode along with a progressing thinning of the ionomer layer might first appear advantageous for the overall performance, but a locally significant decrease in the ionic conductivity may occur as the thinning increases or complete loss of ionomer coverage occurs. This results in a reduction of the effectiveness, as described by Eikerling.57 The platinum is always covered by a thin water layer; electrically connected Pt particles would be visible in voltammetry analysis and would contribute to the electrochemically active surface area (ESCA). However, the exposed Pt particles would be prone to drying by water evaporation and also flooding under wet conditions. Due to the increased transport resistance of the protons without a sufficient amount of ionomer under dry conditions, the contribution to the reaction (and performance) would be significantly reduced. The efforts to reduce the platinum loading in the electrodes may further promote the ionomer degradation at the anode as well as at the cathode. In the anode, a decreased Pt concentration would diminish the local consumption of oxygen permeated through the membrane upon


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reaction at platinum particles, and oxygen may penetrate deeper into the electrode. However, a reduction in the Pt-density increases the mean free path of radicals, and their longer lifetimes further promote thinning of the ionomer layer.58 At the cathodes high potential only small amounts of hydroxyl radicals can form.52 The usual higher platinum loading provides a shorter mean free path between the platinum particles, which reduces the radical attack. A platinum loading that is double at the cathode of the commercial MEA might therefore be an additional reason for the much decreased thinning effect in comparison with the experimental MEA I, in which ionomer layer thinning on the cathode side also occurred. Since the cathode mainly determines the performance of the fuel cell, the minor thinning of the ionomer layers in the commercial MEA might be the reason for the comparably small voltage degradation. In addition to ionomer degradation, other factors likely lead to the thinning of the ionomer layer in the electrode, such as the pressure- or temperature induced flow of ionomer from positions around the Pt/C particles to the pores, or a restructuring of the ionomer layer from a micelle-like structure to a layered structure with a smaller vertical extension. A detailed analysis of the changes in the electrodes after operation will be further performed. .

Conclusion Using material-sensitive and conductive AFM, the structures of fuel cell electrode cross-sections were investigated. The distribution and sizes of Pt/C particles/agglomerates and the thicknesses of the ionomer layers around the Pt/C particles/agglomerates were evaluated. The measurements were performed under humid, close-to-operational conditions to avoid polymer shrinkage. The ionomer portion was easily distinguished by its high adhesion in contrast to the Pt/C-covered area. The agglomeration of Pt/C particles and the separate ionomer shells around these agglomerations were observed. In the electrode cross-sectional area, the average ionomer content was determined as approximately 50%, which was determined from the high-adhesive and nonconductive area. The higher evaluated ionomer content (compared to a nominal ionomer content of 30-40% and resulting area coverage of 35%) was explained by the swelling of the ionomer at high humidity levels and an inhomogeneous ionomer distribution, i.e., from encasement of the Pt/C agglomerates by thin ionomer films. The percent ionomer coverage of the agglomerates was determined by evaluating their conductive area. An ionomer coverage of approximately 50% was determined, in agreement with previous reports from literature. Histograms of the thicknesses of


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ionomer layers around the Pt/C particles/agglomerates revealed a distribution ranging from 4 nm to 20 nm. The average layer thickness before fuel cell operation was different for the three differently prepared MEAs and ranged from 7 to 13 nm. Taking into account the modeling of the structure of the thin Nafion® layers on different substrates and the results from the preparation of the ultra-thin layers, in which a minimum thickness of approximately 4-5 nm was found for lamellar coverage, the thinner ionomer layers were estimated to have a mostly lamellar structure. Thin lamellar films provide reduced oxygen diffusion resistance through the film and low current resistance, allowing current to flow along the Pt-covered agglomerates. After the operation of two MEAs for 235 h in humid conditions, the anode and cathode ionomer layer thicknesses were evaluated again. An increased ionomer thinning at the anode side was observed and could be explained by attacks from oxygen- and hydrogen-based radicals that were generated primarily at the anode in the presence of oxygen and hydrogen gas. Upon heating up to 75°C (near fuel-cell operational temperatures) and cooling, a residual extension of the layer thickness of 25% was observed, which agreed with previous results. The thickness hysteresis is connected with mechanical stress and may promote unfavorable structure changes in the electrodes, an influence that favors electrodes with thinners ionomer layers concerning degradation rate. The correlation of the MEAs with thinner layers having better durability indicated an advantage for fuel cells with a thin lamellar ionomer structure in the electrodes. Thinner membranes and the reduction of platinum loading in the electrodes were identified as promoting factors for the degradation of the electrode ionomer layers. Thinner membranes allow more gas permeation leading to higher radical concentrations, and a decrease in the Pt particle density leads to a larger mean free path for the radials in the ionomer between platinum particles.

ASSOCIATED CONTENT Supporting Information Available: Experimental procedure of AFM calibration data; adhesion force and stiffness values retrieved by AFM for Nafion® and Aquivion® PFSA ultra-thin layers with different thicknesses deposited on Pt sputter layer and on highly oriented pyrolytic graphite, Pt-particles, Pt sputter layer, Vulcan XC72 layer; AFM adhesion images of Pt particle/ionomer layer, Pt particles on ionomer, redeposited Pt particles embedded in ionomer membrane, electrode-membrane interface of operated membrane-electrode assembly (MEA).


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This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements The authors thank Dr. Jürgen Kraut for the SEM analysis and Nadine Hoffmann for the data evaluation. T.M., C.O., K.A.F., and R.H. received funding from the European Union’s Seventh Framework Programme (FP7/2007-2013) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement n°303452, “IMPACT— Improved Lifetime of Automotive Application Fuel Cells with ultra-low Pt-loading”, the authors T.M., M.H., K.A.F., and R.H. received funding from the European Union’s Seventh Framework Programme (FP7/2007-2013) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement n° 621237, “INSIDE- In-situ Diagnostics in Water Electrolyzers”.

Corresponding Author E-mail: *[email protected]

Notes The authors declare no competing financial interest.

REFERENCES (1)

Gago, A. S.; Esquivel, J.-P.; Sabaté, N.; Santander, J.; Alonso-Vante, N. Comprehensive Characterization and Understanding of Micro-Fuel Cells Operating at High Methanol Concentrations. Beilstein J. Nanotechnol. 2015, 6, 2000–2006. (2) Chen, Z.; Higgins, D.; Yu, A.; Zhang, L.; Zhang, J. A Review on Non-Precious Metal Electrocatalysts for PEM Fuel Cells. Energy Env. Sci 2011, 4 (9), 3167–3192. (3) Charreteur, F.; Ruggeri, S.; Jaouen, F.; Dodelet, J. P. Increasing the Activity of Fe/N/C Catalysts in PEM Fuel Cell Cathodes Using Carbon Blacks with a High-Disordered Carbon Content. Electrochimica Acta 2008, 53 (23), 6881–6889. (4) Mani, P.; Srivastava, R.; Strasser, P. Dealloyed Pt-Cu Core-Shell Nanoparticle Electrocatalysts for Use in PEM Fuel Cell Cathodes. J. Phys. Chem. C 2008, 112 (7), 2770–2778. (5) Wells, P. P.; Qian, Y.; King, C. R.; Wiltshire, R. J. K.; Crabb, E. M.; Smart, L. E.; Thompsett, D.; Russell, A. E. To Alloy or Not to Alloy? Faraday Discuss. 2008, 138 (0), 273–285. (6) Izhar, S.; Yoshida, M.; Nagai, M. Characterization and Performances of Cobalt–tungsten and Molybdenum–tungsten Carbides as Anode Catalyst for PEFC. Electrochimica Acta 2009, 54 (4), 1255–1262. (7) Eikerling, M.; Kulikovsky, A. A. Polymer Electrolyte Fuel Cells: Physical Principles of Materials and Operation; CRC Press, Taylor & Francis Group, CRC Press is an imprint of the Taylor & Francis Group, an informa business: Boca Raton, 2015. (8) Sadeghi, E.; Putz, A.; Eikerling, M. Effects of Ionomer Coverage on Agglomerate Effectiveness in Catalyst Layers of Polymer Electrolyte Fuel Cells. J. Solid State Electrochem. 2014, 18 (5), 1271–1279. (9) Iden, H.; Takaichi, S.; Furuya, Y.; Mashio, T.; Ono, Y.; Ohma, A. Relationship between Gas Transport Resistance in the Catalyst Layer and Effective Surface Area of the Catalyst. J. Electroanal. Chem. 2013, 694, 37–44. (10) Weber, A. Z.; Newman, J. Modeling Transport in Polymer-Electrolyte Fuel Cells. Chem. Rev. 2004, 104 (10), 4679–4726.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 40

(11) Kongkanand, A.; Mathias, M. F. The Priority and Challenge of High-Power Performance of Low-Platinum Proton-Exchange Membrane Fuel Cells. J. Phys. Chem. Lett. 2016, 7 (7), 1127–1137. (12) Iden, H.; Ohma, A. An in Situ Technique for Analyzing Ionomer Coverage in Catalyst Layers. J. Electroanal. Chem. 2013, 693, 34–41. (13) Holdcroft, S. Fuel Cell Catalyst Layers: A Polymer Science Perspective. Chem. Mater. 2014, 26 (1), 381–393. (14) Sun, C.-N.; More, K. L.; Veith, G. M.; Zawodzinski, T. A. Composition Dependence of the Pore Structure and Water Transport of Composite Catalyst Layers for Polymer Electrolyte Fuel Cells. J. Electrochem. Soc. 2013, 160 (9), F1000–F1005. (15) Berejnov, V.; Susac, D.; Stumper, J.; Hitchcock, A. P. 3D Chemical Mapping of PEM Fuel Cell Cathodes by Scanning Transmission Soft X-Ray SpectroTomography. ECS Trans. 2013, 50 (2), 361–368. (16) Lopez-Haro, M.; Guétaz, L.; Printemps, T.; Morin, A.; Escribano, S.; Jouneau, P.-H.; Bayle-Guillemaud, P.; Chandezon, F.; Gebel, G. Three-Dimensional Analysis of Nafion Layers in Fuel Cell Electrodes. Nat Commun 2014, 5. (17) More, K. L.; Borup, R.; Reeves, K. S. Identifying Contributing Degradation Phenomena in PEM Fuel Cell Membrane Electrode Assemblies via Electron Microscopy. ECS Trans. 2006, 3 (Copyright (C) 2015 American Chemical Society (ACS). All Rights Reserved.), 717–733. (18) Borup, R.; Meyers, J.; Pivovar, B.; Kim, Y. S.; Mukundan, R.; Garland, N.; Myers, D.; Wilson, M.; Garzon, F.; Wood, D.; Zelenay, P.; More, K.; Stroh, K.; Zawodzinski, T.; Boncella, J.; McGrath, J. E.; Inaba, M.; Miyatake, K.; Hori, M.; Ota, K.; Ogumi, Z.; Miyata, S.; Nishikata, A.; Siroma, Z.; Uchimoto, Y.; Yasuda, K.; Kimijima, K.; Iwashita, N. Scientific Aspects of Polymer Electrolyte Fuel Cell Durability and Degradation. Chem. Rev. 2007, 107 (10), 3904–3951. (19) Suzuki, A.; Sen, U.; Hattori, T.; Miura, R.; Nagumo, R.; Tsuboi, H.; Hatakeyama, N.; Endou, A.; Takaba, H.; Williams, M. C.; Miyamoto, A. Ionomer Content in the Catalyst Layer of Polymer Electrolyte Membrane Fuel Cell (PEMFC): Effects on Diffusion and Performance. Int. J. Hydrog. Energy 2011, 36 (3), 2221–2229. (20) Paul, D.; Fraser, A.; Pearce, J.; Karan, K. Understanding the Ionomer Structure and the Proton Conduction Mechanism in PEFC Catalyst Layer: Adsorbed Nafion on Model Substrate; 2011; pp 1393–1406. (21) Paul, D. K.; Fraser, A.; Karan, K. Towards the Understanding of Proton Conduction Mechanism in PEMFC Catalyst Layer: Conductivity of Adsorbed Nafion Films. Electrochem. Commun. 2011, 13 (8), 774–777. (22) Paul, D. K.; Karan, K.; Docoslis, A.; Giorgi, J. B.; Pearce, J. Characteristics of SelfAssembled Ultrathin Nafion Films. Macromolecules 2013, 46 (9), 3461–3475. (23) Damasceno Borges, D.; Franco, A. A.; Malek, K.; Gebel, G.; Mossa, S. Inhomogeneous Transport in Model Hydrated Polymer Electrolyte Supported Ultrathin Films. ACS Nano 2013, 7 (8), 6767–6773. (24) Artyushkova, K.; Atanassov, P.; Dutta, M.; Wessel, S.; Colbow, V. Structural Correlations: Design Levers for Performance and Durability of Catalyst Layers. J. Power Sources 2015, 284, 631–641. (25) Mayrhofer, K. J. J.; Arenz, M. 7 - Catalyst Ageing and Degradation in Polymer Electrolyte Membrane Fuel Cells. In Polymer Electrolyte Membrane and Direct Methanol Fuel Cell Technology; Hartnig, C., Roth, C., Eds.; Woodhead Publishing, 2012; Vol. 1, p 178–194e. (26) Chen, B.; Ke, W.; Luo, M.; Wang, J.; Tu, Z.; Pan, M.; Zhang, H.; Liu, X.; Liu, W. Operation Characteristics and Carbon Corrosion of PEMFC (Proton Exchange Membrane Fuel Cell) with Dead-Ended Anode for High Hydrogen Utilization. Energy 2015, 91, 799– 806. (27) Hitchcock, A. P.; Berejnov, V.; Lee, V.; West, M.; Colbow, V.; Dutta, M.; Wessel, S. Carbon Corrosion of Proton Exchange Membrane Fuel Cell Catalyst Layers Studied by Scanning Transmission X-Ray Microscopy. J. Power Sources 2014, 266, 66–78. (28) Front Matter A2 - Mench, Matthew M. In Polymer Electrolyte Fuel Cell Degradation; Kumbur, E. C., Veziroglu, T. N., Eds.; Academic Press: Boston, 2012; p iii. (29) Economou, N. J.; O’Dea, J. R.; McConnaughy, T. B.; Buratto, S. K. Morphological Differences in Short Side Chain and Long Side Chain Perfluorosulfonic Acid Proton Exchange Membranes at Low and High Water Contents. RSC Adv. 2013, 3 (42), 19525. (30) Morawietz, T.; Handl, M.; Simolka, M.; Friedrich, K. A.; Hiesgen, R. Membranes, Electrodes, and Membrane-Electrodes Assemblies Analyzed before and after Operation by Atomic Force Microscopy. ECS Trans. 2015, 68 (3), 3–12.


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(31) Hiesgen, R.; Morawietz, T.; Handl, M.; Corasaniti, M.; Friedrich, K. A. Atomic Force Microscopy on Cross Sections of Fuel Cell Membranes, Electrodes, and Membrane Electrode Assemblies. Electrochimica Acta 2015, 162, 86–99. (32) Hiesgen, R.; Helmly, S.; Galm, I.; Morawietz, T.; Handl, M.; Friedrich, K. A. Microscopic Analysis of Current and Mechanical Properties of Nafion® Studied by Atomic Force Microscopy. Membranes 2012, 2 (4), 783–803. (33) Sasikumar, G.; Ihm, J. W.; Ryu, H. Optimum Nafion Content in PEM Fuel Cell Electrodes. Electrochimica Acta 2004, 50 (2–3), 601–605. (34) Passalacqua, E.; Lufrano, F.; Squadrito, G.; Patti, A.; Giorgi, L. Nafion Content in the Catalyst Layer of Polymer Electrolyte Fuel Cells: Effects on Structure and Performance. Electrochimica Acta 2001, 46 (6), 799–805. (35) Yu, H.; Roller, J. M.; Mustain, W. E.; Maric, R. Influence of the Ionomer/carbon Ratio for Low-Pt Loading Catalyst Layer Prepared by Reactive Spray Deposition Technology. J. Power Sources 2015, 283, 84–94. (36) Moore, M.; Wardlaw, P.; Dobson, P.; Boisvert, J. J.; Putz, A.; Spiteri, R. J.; Secanell, M. Understanding the Effect of Kinetic and Mass Transport Processes in Cathode Agglomerates. J. Electrochem. Soc. 2014, 161 (8), E3125–E3137. (37) Owejan, J. P.; Owejan, J. E.; Gu, W. Impact of Platinum Loading and Catalyst Layer Structure on PEMFC Performance. J. Electrochem. Soc. 2013, 160 (8), F824–F833. (38) Ikeda, K.; Nonoyama, N.; Ikogi, Y. Analysis of the Ionomer Coverage of Pt Surface in PEMFC; 2010; pp 1189–1197. (39) Damasceno Borges, D.; Gebel, G.; Franco, A. A.; Malek, K.; Mossa, S. Morphology of Supported Polymer Electrolyte Ultrathin Films: A Numerical Study. J. Phys. Chem. C 2015, 119 (2), 1201–1216. (40) Ohira, A.; Kuroda, S.; Mohamed, H. F. M. A Study on Structural Property of Ionomer as a Model for Catalyst Layer: Relationship between Thickness and Proton Conduction for Ionomer Thin Film on Different Substrate. ECS Trans. 2013, 50 (2), 993–1001. (41) Kalisvaart, W. P.; Fritzsche, H.; Merida, W. Water Uptake and Swelling Hysteresis in a Nafion Thin Film Measured with Neutron Reflectometry. Langmuir 2015, 31, 5416–5422. (42) Lee, D.; Hwang, S. Effect of Loading and Distributions of Nafion Ionomer in the Catalyst Layer for PEMFCs. Int. J. Hydrog. Energy 2008, 33 (11), 2790–2794. (43) Park, Y.-C.; Kakinuma, K.; Uchida, H.; Watanabe, M.; Uchida, M. Effects of Short-SideChain Perfluorosulfonic Acid Ionomers as Binders on the Performance of Low Pt Loading Fuel Cell Cathodes. J. Power Sources 2015, 275, 384–391. (44) Xie, J.; Xu, F.; Wood III, D. L.; More, K. L.; Zawodzinski, T. A.; Smith, W. H. Influence of Ionomer Content on the Structure and Performance of PEFC Membrane Electrode Assemblies. Electrochimica Acta 2010, 55 (24), 7404–7412. (45) Gatto, I.; Stassi, A.; Baglio, V.; Carbone, A.; Passalacqua, E.; Aricò, A. S.; Schuster, M.; Bauer, B. Optimization of Perfluorosulphonic Ionomer Amount in Gas Diffusion Electrodes for PEMFC Operation under Automotive Conditions. Electrochimica Acta 2015, 165, 450–455. (46) Suzuki, A.; Sen, U.; Hattori, T.; Miura, R.; Nagumo, R.; Tsuboi, H.; Hatakeyama, N.; Endou, A.; Takaba, H.; Williams, M. C.; Miyamoto, A. Ionomer Content in the Catalyst Layer of Polymer Electrolyte Membrane Fuel Cell (PEMFC): Effects on Diffusion and Performance. Int. J. Hydrog. Energy 2011, 36 (3), 2221–2229. (47) Masuda, T.; Naohara, H.; Takakusagi, S.; Singh, P. R.; Uosaki, K. Formation and Structure of Perfluorosulfonated Ionomer Thin Film on a Graphite Surface. Chem. Lett. 2009, 38 (9), 884–885. (48) De Moor, G.; Bas, C.; Charvin, N.; Dillet, J.; Maranzana, G.; Lottin, O.; Caqué, N.; Rossinot, E.; Flandin, L. Perfluorosulfonic Acid Membrane Degradation in the Hydrogen Inlet Region: A Macroscopic Approach. Int. J. Hydrog. Energy 2016, 41 (1), 483–496. (49) Chen, C.; Fuller, T. F. The Effect of Humidity on the Degradation of Nafion® Membrane. Polym. Degrad. Stab. 2009, 94 (9), 1436–1447. (50) Sethuraman, V. A.; Weidner, J. W.; Haug, A. T.; Protsailo, L. V. Durability of Perfluorosulfonic Acid and Hydrocarbon Membranes: Effect of Humidity and Temperature. J. Electrochem. Soc. 2008, 155 (2), B119–B124. (51) Bi, W.; Sun, Q.; Deng, Y.; Fuller, T. F. The Effect of Humidity and Oxygen Partial Pressure on Degradation of Pt/C Catalyst in PEM Fuel Cell. Electrochimica Acta 2009, 54 (6), 1826–1833. (52) Antoine, O.; Durand, R. RRDE Study of Oxygen Reduction on Pt Nanoparticles inside Nafion®: H2O2 Production in PEMFC Cathode Conditions. J. Appl. Electrochem. 2000, 30 (7), 839–844.


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(53) Coms, F. D. The Chemistry of Fuel Cell Membrane Chemical Degradation; ECS Trans. 2008, 16, 235–255. (54) Chen, S.; Gasteiger, H. A.; Hayakawa, K.; Tada, T.; Shao-Horn, Y. Platinum-Alloy Cathode Catalyst Degradation in Proton Exchange Membrane Fuel Cells: NanometerScale Compositional and Morphological Changes. J. Electrochem. Soc. 2010, 157 (1), A82. (55) Ramaswamy, N.; Hakim, N.; Mukerjee, S. Degradation Mechanism Study of Perfluorinated Proton Exchange Membrane under Fuel Cell Operating Conditions. Electrochimica Acta 2008, 53 (8), 3279–3295. (56) Mu, S.; Xu, C.; Yuan, Q.; Gao, Y.; Xu, F.; Zhao, P. Degradation Behaviors of Perfluorosulfonic Acid Polymer Electrolyte Membranes for Polymer Electrolyte Membrane Fuel Cells under Varied Acceleration Conditions. J. Appl. Polym. Sci. 2013, 129 (3), 1586–1592. (57) Chan, K.; Eikerling, M. A Pore-Scale Model of Oxygen Reduction in Ionomer-Free Catalyst Layers of PEFCs. J. Electrochem. Soc. 2011, 158 (1), B18. (58) Helmly, S.; Ohnmacht, B.; Gazdzicki, P.; Hiesgen, R.; Gülzow, E.; Friedrich, K. A. Influence of the Distribution of Platinum Deposits on the Properties and Degradation of Platinum-Impregnated Nafion Membranes. J. Electrochem. Soc. 2014, 161 (14), F1416– F1426.


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Graphic for manuscript

TOC-Scheme: Quantitative evaluation of ionomer layers in catalytic fuel cell electrodes, before and after operation


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Scheme 1. AFM measurement of a MEA cross-section sample. Scheme 1 212x123mm (150 x 150 DPI)


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Figure 1. a) AFM 3D-topography of a cross-section of the Aquivion® PFSA-based experimental MEA I. b) AFM 3D-topography of a) overlaid with current recorded at U= 100 mV, images measured at RT and 3040% RH. Figure 1 245x133mm (150 x 150 DPI)



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Figure 2. AFM images of a cross-section of the cathode of the Nafion®-based commercial MEA, measured at RT and 50-60% RH. a) The topography image. b) The adhesion image, in which brighter colors indicate higher adhesion and refer to the ionomer; black/brown refers to the platinum and carbon, respectively. c) The stiffness (DMT modulus) mapping. d) A 3-dimensional view of the electrode surface overlaid with the adhesion mapping. Figure 2 162x165mm (150 x 150 DPI)


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Figure 3. a) The ionomer coverage of the different MEAs measured at RT and 50-60% RH. b) The current mapping of the anode of the Nafion®-based commercial MEA; the current is marked in blue and the nonconductive area is marked in red (42%). c) The adhesion mapping of the same area as b) with an ionomer coverage of 47%, marked in green. Figure 3 232x79mm (150 x 150 DPI)



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Figure 4. The dependence of the mean Pt/C agglomerate size of the electrodes of three different MEAs, retrieved from adhesion mappings, as measured at 50-60% RH and RT. Figure 4 206x157mm (150 x 150 DPI)


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Figure 5. a) Adhesion mapping of the anode of the commercial MEA. b) The mapping of the conductive areas overlaid on the areas of agglomerates. c) A comparison of the ionomer coverage of agglomerates for all electrodes. Figure 5 240x98mm (150 x 150 DPI)



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Figure 6. a) A high-resolution adhesion mapping of the commercial MEA anode measured at RT and 30-40% RH; the ionomer is marked in blue, and the arrows mark the positions of the separated layers. b) The zoomed-in image of a high-resolution adhesion mapping of the commercial MEA anode with two distinguishable layers around the Pt/C agglomerates measured at RT and 25% RH. c) A cartoon of ionomerenclosed Pt/C agglomerates. Figure 6 243x91mm (150 x 150 DPI)


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Figure 7. AFM images of a cross-section of the cathode of the commercial MEA, as measured at RT and 3040% RH and U=50 mV. a) The adhesion force: Pt-rich area appears dark. b) The corresponding current image with high current (bright) at Pt-rich areas. c) The adhesion force (black) and current profile (blue) measured across the ionomer layer between two particles along the marked lines in a) and b), respectively. Figure 7 141x184mm (150 x 150 DPI)



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Figure 8. A histogram, together with the fitted log-normal curves as the dashed lines, of all ionomer thickness values from two different positions covering an area of 1 µm x 1 µm, measured from the crosssections of a Nafion® -based commercial MEA cathode at RT and 50-60% RH. Figure 8 248x190mm (150 x 150 DPI)


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Figure 9. The dependence of the ionomer layer thickness of the commercial MEA cathode on heating (red curve) and subsequent cooling (blue curve), as measured at 30-40% RH. Figure 9 209x161mm (150 x 150 DPI)



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Figure 10. The relative frequency of the ionomer layer thickness of the experimental MEA II (red bars) anodes with Aquivion® PFSA as the ionomer, the commercial Nafion®-based MEA (green bars), and the experimental Aquivion® PFSA-based MEA I (blue bars). All measurements were performed before operation at RT and 50-60% RH on the cross-sections. Figure 10 256x199mm (150 x 150 DPI)


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Figure 11. The relative frequency of the ionomer layer thickness for the commercial MEA and the experimental MEA I (both samples from hydrogen inlet) before and after 235 h of operation, as measured at 50-60% RH at RT. a) The anode of the commercial MEA. b) The cathode of the commercial MEA. c) The anode of the experimental MEA I. d) The cathode of the experimental MEA I. Figure 11 246x197mm (150 x 150 DPI)



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TOC-Scheme: Quantitative evaluation of ionomer layers in catalytic fuel cell electrodes, before and after operation. TOC 253x110mm (150 x 150 DPI)


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Quantitative in Situ Analysis of Ionomer Structure in Fuel Cell Catalytic

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