Ionomer Distribution Research Articles

Optimization Ionomer Distribution on Porous Carbon Catalyst Support to Commercialize Low Pt Loaded PEMFCS

One of the major drawbacks towards PEMFC (polymer exchange membrane fuel cell) commercialization is the high amount of platinum needed at the cathodic electrode, due to the inherent slow kinetics of the oxygen reduction reaction. It has been widely known that the reduction of the Pt loading at the cathodic electrode leads to significant voltage losses. The origin of which has been correlated to electrode structure, ionomer distribution, type of catalyst support, and location of the Pt particles on the carbon support. A recent study by Orfanidi et al showed that by introducing positively charged nitrogen groups into the carbon structure lead to a more homogeneous distribution of the ionomer, which results in higher performance even at low Pt loaded electrodes [1]. In the present study, a series of different N-functionalized Ketjenblack are synthesized and fully characterized to investigate their use as support material for Pt based electrodes. Pre-oxidized Ketjenblack was heat treated in ammonia at different temperatures [2] followed by Pt precipitation via an ethylenglycol method. This new type of catalyst significantly reduces the additional mass transport resistance and report an unprecedented high coverage of the ionomer over the high surface area carbon supports, especially under dry operating conditions, in addition to high stability under voltage cycling. Those enhanced transport behaviors are deduced to local transport phenomena in the porous carbon structure and we give a tailoring approach for such a carbon material within this work. Our catalyst presents significant advantages as it facilitates manufacturing processes of the electrodes, due to the columbic interaction between the ionomer and the –NHx groups on carbon support, ensuring homogeneous ionomer distribution. This work sheds light into the effect of the different N based groups towards better understanding of the interaction of ionomer with the carbon support. Our concept can be transferred to other kinds of partially amorphous carbon and is not limited to Pt as an active material. Introduction of such a modification step in the production line of electrocatalysts paves the way for PEMFC commercialization in automotive applications, due to enhancement in mass transport and stability. Literature [1] A. Orfanidi, P. Madkikar, H.A. El-Sayed, G.S. Harzer , T. Kratky, H.A. Gasteiger, Journal of the Electrochemical Society 2017, 164, F418-F26. [2] K.F. Ortega, R. Arrigo, B. Frank, R. Schlöten, A. Trunschke, Chemistry of Materials 2016, 28, 6826-6839.

Analysis of PEMFC Electrode Structure – Bridging the Mesoscale Gap

Proton exchange membrane fuel cell (PEMFC) electrodes are commonly composed of Pt nanoparticles supported on conductive carbon interspersed with ionomer, creating a porous network facilitating transport of electrons, protons, both reactant and product gasses, and liquid water. For the electrochemical reactions to occur at the metallic Pt active sites in the cathode, the electrons, protons, and oxygen all need to be present, which presumably requires the confluence of carbon, ionomer, and Pt all accessible at a pore. Following reaction at this 3-phase boundary, the pore network needs to remove product water from the electrode. Though the structure of PEMFC electrode layers has been studied for many years, the details are still unclear. The distribution of pore sizes, thickness and distribution of ionomer, and structure of carbon agglomerates are not well understood. This lack of understanding hinders rational design of catalyst layers that utilize reduced Pt loading while maintaining the performance and durability necessary for large-scale commercialization. Part of the difficulty in obtaining accurate morphological information on these complex structures is that it is unclear what techniques provide accurate analysis of the pore structures within these materials. Common pore size analysis techniques include nitrogen adsorption (BET), mercury intrusion (MIP), x-ray tomography (XCT), transmission electron microscopy (TEM), and focused ion beam tomography (FIB-SEM). When multiple techniques are applied, even to the same sample, the results for pore size distribution and connectivity can vary widely. It is unclear which, if any, of these techniques yield results representative of the true pore structure within the catalyst layer. Further, it is not known if the lack of agreement results from sample preparation methods and analysis conditions used for each technique, or fundamental limitations of the techniques when applied to PEMFC electrode materials. In this work, we examine results of pore analysis from multiple techniques. In addition to standard measurement techniques including BET, TEM, MIP, and FIB-SEM tomography, we employ techniques for measurement of pore size by analysis of the solid/liquid phase transition temperature using cryoporometry (Figure 1). We will discuss the results of these multiple techniques, address their utility and limitations, and attempt to present a pore size distribution in better agreement with reality that can be used as input for modeling efforts. Acknowledgement: This research is supported by the U.S. Department of Energy Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium (Technical Development Manager: Greg Kleen and Fuel Cells Program Manager: Dimitrios Papageoropoulos). Figure 1. Comparison of pore size distributions from nitrogen isotherm analysis using BJH1 (top) and cryoporometry (bottom). Top figure is PSD for TEC10E50E (CB), TEC10EA30E (GCB), and acetylene black supported Pt with specific surface areas of 779 and 219 m2 g-1 (AB800 and AB250 respectively). Bottom figure is PSD for TEC10E50E. 1) Young-Chul Park, Haruki Tokiwa, Katsuyoshi Kakinuma, Masahiro Watanabe, Makoto Uchida, Effects of carbon supports on Pt distribution, ionomer coverage and cathode performance for polymer electrolyte fuel cells, Journal of Power Sources, 315 (2016) pp.179-191 Figure 1

(Invited) Design of Catalyst Layer Structure for High Fuel Cell Performance

The low Pt electrode is an important research topic for the commercialization of polymer electrolyte fuel cell (PEFC). Platinum nanoparticles act as a catalyst for the fuel cell electrode and promote the oxygen reduction reaction (ORR). In this process, the surface properties and structure of adjacent carbon supports and ionomer binders greatly affect the Pt catalyst. Therefore, the performance of the Pt/C catalyst in the fuel cell electrode is determined not only by the intrinsic activity of the catalyst itself, but also by the interface structure between the ionomer binder and the Pt/C catalyst. It has been reported that the ORR activity of the Pt/C catalyst in the fuel cell electrode is affected by the Pt poisoning of the sulfonic acid group tethered to the Nafion backbone as well as the oxygen permeability of the ionomer binder [1]. The difference in pore characteristics of the carbon support affects the optimum ionomer content [2] and Pt utilizaton [3] in the electrode. The surface properties of the carbon support also affect ionomer distribution and ionomer thickness on the catalyst surface, thereby altering the activity of the catalyst [4]. In spite of recent studies, understanding of complicated physicochemical phenomena in the microstructure of electrode is still very limited. In this talk, we will discuss the fuel cell performance characteristics based on several electrochemical analyses according to the microstructure control of the catalyst layer. Various functional groups were introduced on the surface of Pt/C catalyst by post treatment and the catalyst layer was prepared to observe the ionomer distribution and ion transport characteristics according to functional groups. In addition, a multilayer catalyst layer having different ionomer contents in the thickness direction of the catalyst layer was prepared using an ultrasonic spraying process, and changes in hydrogen ion transfer and oxygen diffusion transfer characteristics in these catalyst layers were observed in a fuel cell. Finally, the electrochemical properties of the catalyst layer made of the ionomer dispersed in the high-boiling organic solvent will be introduced. 1. V. Yarlagadda, M. K. Carpenter, T. E. Moylan, R. S. Kukreja, R. Koestner, W. Gu, L. Thompson, A. Kongkanand, ACS Energy Lett. 3 (2018) 618. 2. Y. Liu, C. Ji, W. Gu, J. Jorne, H. A. Gasteiger, J. Electrochem. Soc. 158(6) (2011) B614. 3. Y.-C. Park, H. Tokiwa, K. Kakinuma, M. Watanabe, M. Uchida, J. Power Sources 315 (2016) 179. 4. A. Orfanidi, P. Madkikar, H. A. El-Sayed, G. S. Harzer, T. Kratky, H. A. Gasteiger, J. Electrochem. Soc. 164 (2017) F418.

A Comparative Study of Electrospray and Airbrushing Processes for Deposition of Catalyst Layers

Electrospray and airbrushing are methods for catalyst layer deposition based on spraying a suspension of the ionomer and the catalyst. Airbrushing is most popular because it can be carried out with a simple and cheap set-up, and allows for fast deposition rates with high homogeneity of the resulting layers. In the electrospray deposition, the set-up is a little more complicated because the spray must be produced in the presence of an electric field which confers special characteristics to the deposition process giving rise to different layer properties [1]. This technique allows preparing reproducible and durable catalyst layers with internal superhydrophobic character. When used in the cathode, the electrosprayed layer enhances water back-diffusion from cathode to anode, which improves membrane and anode humidification [2]. Electrosprayed layers also have low mass-transport resistance thanks to their low liquid water content while working in a fuel cell [4]. As a consequence, power conversion increases by around 20% under standard testing conditions, comparing with the conventional airbrushing method. Durability of the cells is also improved by the more homogeneous layer response [2,3]. In this communication the electrospray and airbrushing deposition processes are compared in order to explain the differences observed in the resulting catalyst layers. The electrosprayed film growths by the stacking of catalyst particles and ionomer chains under electrostatic interaction and free of solvent, which is evaporated before arriving solids to the substrate. On the other hand, airbrushing takes place by deposition of suspension droplets, so the catalyst and the ionomer arrive to the substrate together with the solvent and dispose under the same interactions prevailing in the suspension. The different conditions give rise to differences in catalyst and ionomer distribution, morphology, and porosity of the layers. In addition, the electrosprayed catalyst layers are characterized by requiring lower ionomer concentration in the departing ink, typically 15wt% with a Pt/C 20wt% catalyst, which is about half of the amount for an optimal standard airbrushed catalyst layers, and they show a sharp optimization with thickness/loading. Differences in the structure and fuel cell behavior properties will be analyzed in the light of the characteristics of the deposition process.

Enhanced Power Performance of Membrane Electrode Assemblybased on Modified Catalyst Support for PEMFC

The cost reduction and durability enhancement of membrane electrode assembly (MEA) are of primary concerns toward commercialization of fuel cell electric vehicle (FCEV). The electrochemical corrosion of carbon supports is one of the major catalyst degradation modes occurring during loading cycling and startup-shutdown operation for FCEV, and causes a significant influence on the performance loss of MEA.Herein we present the fabrication of durable catalyst supports by the non-covalent modification of the GC support. A simple non-covalent functionalization of the GC support with the pyrene molecules was shown to enhance the dispersion and uniform distribution of Pt nanoparticles with preserving the integrity and electronic structure of carbons [1]. Electrochemical analysis demonstrated that the Pt/modified GC shows improved electrochemical durability with enhanced catalytic activity than the Pt/GC and commercial Pt/GC catalysts. Our study demonstrates that the durability of the Pt/C catalyst depends on the uniformity of Pt nanoparticles dispersion and the capability of corrosion resistance, which can be enhanced by the non-covalent modification of GC.For automotive application, the development of MEA with high power performance and high durability is of much interest. In order to investigate the practical impact of the Pt/modified GC catalyst, we designed two types of MEA employing the Pt/modified GC (Modified GC-MEA) and the Pt/GC (GC-MEA) as the cathode catalysts. The cell voltage of the modified GC-MEA under low humidified condition is higher than that of the GC-MEA in the overall region of current density studied (Fig. 1(a)). The absence of a significant mass transport limitation at high current densities for the modified GC-MEA indicates that the Pt/modified GC provides a Pt surface area large enough for preventing the local oxygen transport limitation. We hypothesized that the possible interaction between modified GC surfaces and ionomer would have an influence on ionomer distribution in the catalyst layer. As shown in Figure 1(b), the modified GC-MEA exhibits lower O2 gain than the GC-MEA under low humidified condition. Lower O2 transport resistance for the modified GC-MEA was ascribed to better O2 gas diffusion in the cathode catalyst layer. To assess the durability of the modified GC-MEA, we conducted the catalyst support durability test, following the standard test protocols suggested by DOE. The voltage loss after 5,000 cycles was about 24 mV for the modified GC-MEA, which was similar to that of the GC-MEA. Our results suggest that the non-covalent functionalization of the GC with the pyrene molecules is an effective way to fabricate highly active and durable catalysts for PEMFC, which can substantially enhance the power performance of MEA under low humidified condition without sacrificing the electrochemical stability of the GC support, providing a design guide of highly durable MEA for automotive applications. [1] H.S. Oh, H. Kim, Adv. Funct. Mater. 21 (2011) 3954-3960. Figure 1

Recent Advances with Sulfonated Silica Ceramic Carbon Electrodes for Fuel Cells

Sulfonated silica-based ceramic carbon electrodes (SS-CCE) are prepared using low-cost organosilane precursors that are mixed with the carbon-supported platinum (Pt/C) catalyst in monomer form, and subsequently polymerized and coated onto a gas diffusion layer in a single step. Our lab has demonstrated that that SS-CCEs can achieve similar H2/O2 fuel cell performance as conventional Nafion-based electrodes [1, 2]. Furthermore, these electrode structures display excellent tolerance to dry operating conditions, due to the proton conductivity of the sulfonic acid moiety and the hygroscopic nature of the SiO2 backbone. This is a major advantage over conventional Nafion-based fuel cell electrodes which require the use of heavily humidified gas feeds and are limited to temperature below 80 °C. Therefore SS-CCEs have great potential to improve PEMFC efficiency by reducing parasitic power losses required for gas humidification. Furthermore, they have the potential to reduce the required catalyst loading and extend the operating temperature range of PEMFCs. While these results have been promising, there are few detailed studies of SS-CCE structure-property. Likewise, their durability in operating fuel cells have yet to be proven. In this presentation, we will give an overview of our group’s recent progress towards optimization the synthesis and structure of SS-CCEs. In particular, we have found hydrothermal conditions to be a promising way to enhance performance, promoting a more homogenous distribution of silicate ionomer and more effective bonding to the carbon surface [3]. Likewise, accelerated stress tests indicate that the silicate ionomer also serves to stability the Pt catalysts and enhance its durability [4]. This, along with their tolerance to high temperatures/low relative humidity conditions leads to improved stability of SS-CCE-based fuel cell systems.

(Invited) Development of a Multiscale Approach for Modeling Fuel Cell Electrodes

Porous electrodes form a critical part of electrochemical energy systems such as polymer electrolyte fuel cells (PEFCs). In order to improve their performance and reduce cost, novel designs for porous electrodes are being envisioned, e.g., non-PGM catalyst layers, ultra-thin Pt/Pt-Ni based catalyst layers, or NSTF catalyst layers.1 These electrodes have features spanning multiple length scales: from nano-scale catalyst particles to micrometer thick catalyst layers; with meso-scale porous structure in between. In order to understand the impact of these new designs on cell performance, a detailed understanding of the multi-scale structure is required. Furthermore, the physical processes at each length scale must be understood and up-scaled in order to evaluate the macro-scale cell performance. The aim of this work is to develop multi-scale models which can account for the effect of micro and meso scale structure on macro-scale performance. Figure 1 shows a schematic of the multi-scale approach for modeling PEM fuel cells. This approach can be used for conventional Pt based electrodes, or for non-PGM based electrodes where large catalyst particles induce significant local flooding and reactant losses. The microstructure is generated using stochastic techniques.2,3 The crucial micro-structural properties such as pore sizes, ECSA, phase distributions are obtained using physical characterization methods such as imaging, Cyclic voltammetry, and BET. The generated microstructures will be used to simulate pore-level physics, where local transport, and reactions will be modeled. The pore scale performance will then be upscaled and used with macro-scale models to simulate system level performance. To analyze critical parameters for fuel cell performance, sensitivity studies will be performed using the macro-scale model. The aim is to identify parameters which affect the cell performance significantly and therefore must be measured accurately and provide a way to optimize cell performance. Apart from macro-scale parameters, the multi-scale sensitivity study can also be used to optimize ionomer and catalyst distribution. Acknowledgements The work is funded under the Fuel Cell Performance and Durability Consortium (FC-PAD), by the Fuel Cell Technologies Office (FCTO), Office of Energy Efficiency and Renewable Energy (EERE), of the U.S. Department of Energy under contract number DE-AC02-05CH11231. References P. K. Sinha, W. Gu, A. Kongkanand and E. Thompson, J. Electrochem. Soc., 158, B831 (2011).F. C. Cetinbas, R. K. Ahluwalia, N. N. Kariuki and D. J. Myers, J. Electrochem. Soc., 165, F1051 (2018).L. M. Pant, S. K. Mitra and M. Secanell, Phys. Rev. E, 92, 063303 (2015). Figure 1

Electrospun Particle/Polymer Fiber Electrodes with a Neat Nafion Binder for Hydrogen/Air Fuel Cells

Membrane-electrode-assemblies with electrospun particle/polymer fiber electrodes produce high power at low Pt loading in a proton exchange membrane H2/air fuel cell, with improved cathode durability as compared to conventional sprayed electrodes [1-5]. This boost in performance has been attributed to inter and intra fiber porosity. The increased porosity of a fiber mat cathode allows for facile removal of product water during fuel cell operation. Porosity within a fiber improves access of reactant gases to catalyst sites and may assist in retaining water during low relative humidity fuel cell operation. Additionally, shear mixing of catalyst and binder during the process of creating an electrospun fiber results in a more uniform distribution of ionomer and catalyst with few, if any, polymer or catalyst aggregates. Previous work on fiber electrodes focused on the use of poly(acrylic acid) (PAA) [1-3, 5] or poly(vinylidene fluoride) (PVDF) [4, 5] as a carrier polymer for Nafion perfluorosulfonic acid during electrospinning. In the present work, poly(ethylene oxide) (PEO) was used as the electrospinning carrier polymer for the fabrication of fiber mat cathodes and anodes, with either Pt/C or PtCo/C as the cathode catalyst and Pt/C as the anode catalyst. PEO carrier polymer was washed out of the fiber mat, resulting in a neat Nafion binder. For a fixed cathode and anode Pt loading of 0.1 mg/cm2, fuel cell tests were performed at 80 °C and 200 kPaabs pressure. In this talk, the effect of carrier polymer on fiber morphology will be discussed using STEM-EDX data collected at Oak Ridge National Laboratory. Beginning-of-life fuel cell power output was measured at feed gas relative humidities from 40% to 100%. The fuel cell polarization results, along with cathode ECSA and mass activity data were contrasted with conventional spray electrode MEAs. Fiber electrode MEAs with Pt/C or PtCo/C cathodes produced very high power at high and low feed gas RH. For example, the maximum power for a Pt/C cathode was essentially constant at 830 mW/cm2 ±5% (experimental error) from 40% RH to 100% RH. Additionally, fiber cathode MEAs exhibited better durability in a load cycling accelerated stress test, e.g., fiber electrode MEAs made with a Nafion/PEO electrospinning ink typically retained ~80% of their initial maximum power vs. 70% retention for a sprayed electrode MEA. This talk will focus on: (i) the methods for fiber electrode electrospinning, (ii) the morphology of the resulting fibers, and (iii) contrasting the performance of fiber electrode MEAs with sprayed electrode MEAs. Acknowledgements This research is supported by the U.S. Department of Energy Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium (Fuel Cells Program Manager: Dimitrios Papageorgopoulos). The work at Vanderbilt University was funded under DOE contract No. DE-EE0007653. References W. Zhang and P. N. Pintauro, ChemSusChem, 4, 1753-1757 (2011).M. Brodt, R. Wycisk, and P. N. Pintauro, J. Electrochem. Soc., 160, F744-F749 (2013).M. Brodt, T. Han, N. Dale, E. Niangar, R. Wycisk, and P. Pintauro, J. Electrochem. Soc., 162, F84-F91 (2015).M. Brodt, R. Wycisk, N. Dale, and P. Pintauro, J. Electrochem. Soc., 163, F401-F410 (2016).J. J. Slack, R. Wycisk, N. Dale, A. Kumar, and P. N. Pintauro, J.Electrochem. Transactions, 80, 829-837 (2017).

In-Situ Electrochemical Techniques to Determine Ionomer Coverage in PEFC Electrodes

Many electrochemical devices such as low temperature fuels utilize ionomers to improve efficiency and overall performance. In polymer electrolyte fuel cells (PEFCs), perflurosulfonic acid ionomers improve ionic (protonic) conductivity and Pt utilization within the catalyst layer. However, the strong interaction between sulfonate (-SO3 -) functional groups and Pt nanoparticles have been linked lower catalytic rates and higher O2 transport resistances.1,2 There have been numerous strategies implemented to control the local distribution of ionomer and Pt/C catalyst particles, but limitations in the resolution of state-of-the-art characterization techniques make it difficult to assess how ink formulations and/or electrode fabrication techniques control ionomer-catalyst interactions and its impact device performance.3–5 Herein, the development and utilization of in situ electrochemical techniques designed to probe the local ionomer interactions will be utilized in conjunction with established testing protocols to relate ionomer coverage (on Pt and/or C support) with kinetic performance and O2 transport resistance of fully-conditioned PEFC electrodes. Specifically, CO displacement chronoamperometry and electrochemical impedance spectroscopy techniques will be described to determine local ionomer coverages across a series of Pt/C MEAs. These results will promote a deeper understanding of ionomer-catalyst interactions which in turn will inform future materials development and integration necessary to achieve high performance PGM (e.g. Pt/C and Pt alloy/C) electrocatalyst-based electrodes. Schuler, T. et al. Fuel-Cell Catalyst-Layer Resistance via Hydrogen Limiting-Current Measurements. J. Electrochem. Soc. 166, F3020–F3031 (2019).Subbaraman, R., Strmcnik, D., Paulikas, A. P., Stamenkovic, V. R. & Markovic, N. M. Oxygen Reduction Reaction at Three-Phase Interfaces. ChemPhysChem 11, 2825–2833 (2010).Berlinger, S. A., McCloskey, B. D. & Weber, A. Z. Inherent Acidity of Perfluorosulfonic Acid Ionomer Dispersions and Implications for Ink Aggregation. J. Phys. Chem. B 122, 7790–7796 (2018).Orfanidi, A., Rheinländer, P. J., Schulte, N. & Gasteiger, H. A. Ink Solvent Dependence of the Ionomer Distribution in the Catalyst Layer of a PEMFC. J. Electrochem. Soc. (2018). doi:10.1149/2.1251814jesKhandavalli, S. et al. Rheological Investigation on the Microstructure of Fuel Cell Catalyst Inks. ACS Appl. Mater. Interfaces (2018). doi:10.1021/acsami.8b15039

Microscopic Analysis of PEMFC Catalyst Layers

Proton exchange membrane fuel cell (PEMFC) electrodes have a complex structure of carbon supported platinum (Pt) nanoparticles intermixed with proton conducting ionomer. This structures creates a porous network facilitating transport of electrons, protons, and reactants. These catalyst layers have been shown to exist with a non-homogenous distribution of ionomer, with aggregates and agglomerates of carbon and ionomer. To better understand the catalyst layer structure, we have applied atomic force microscopy (AFM) to investigate fuel cell catalyst layer properties at high resolution. Various ionomer-carbon (I/C) ratios were analyzed in order to identify correlations between carbon agglomerate size, pore distribution, and structure. Surface and cross-sectional images of topography, adhesion, and roughness were collected. Scanning electron microscopy (SEM) was performed to allow for additional depictions of the samples. Several other techniques are planned to further explore structures. Electrochemical capabilities of the AFM will be utilized to inspect the effects of current on the aforementioned properties and provide better control in measurements. Additionally, samples will be placed in a water-filled holder to study effects of swelling through in situ AFM. Transmission electron microscopy (TEM) will also be performed to complement these measurements. A Bruker AFM was used in PeakForce tapping mode with a ScanAsyst Air tip to obtain the images in Figure 1. Figure 1a shows the height map of a sample with an I/C ratio of 1. Figure 1b is the adhesion map of the same sample, showing clear spatial adhesion contrast. In this map, lighter colors correspond to the more adhesive ionomer. Darker areas signify hard materials like carbon. Figure 1c depicts the adhesion map overlaid on the height map, indicating how the adhesion and topography relate. The arrows in Figure 1c illustrate where the ionomer thickness can be measured between carbon aggregates. Figure 2a summarizes the average ionomer thickness calculated for a range of I/C ratios from 0.25 to 1.25. There is a positive correlation between I/C ratio and average ionomer thickness, suggesting that increased availability of ionomer leads to thicker surface coatings and more separation between the carbon aggregates. Figure 2b shows the relationship between I/C ratio and aggregate diameter. These values were measured by processing the images in ImageJ and reducing aggregates to approximated spheres. Although no clear relationship is apparent now, further investigation with differing I/C ratios may reveal a trend. Acknowledgements : This research is supported by the U.S. Department of Energy Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium (Technical Development Manager: Greg Kleen and Fuel Cells Program Manager: Dimitrios Papageoropoulos). This work was also performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Figure 1

(Invited) Structural Characterization of Polymer Electrolyte Fuel Cell Electrodes Using Nano-Scale Synchrotron X-Ray Computed Tomography

With improvements in the intrinsic oxygen reduction reaction activity of both Pt-based and platinum group metal-free (PGM-free) catalysts for polymer electrolyte fuel cells, in recent years more attention is being focused on utilization of the active sites in these catalysts under conditions relevant to the automotive application. Taking full advantage of the performance offered by these high activity catalysts requires comprehensive knowledge of the electrode microstructure and the spatial distribution of the catalyst, support, ionomer, and pores. Nano-scale resolution X-ray computed tomography (nano-CT) has been used as a powerful tool for non-destructive quantification of three-dimensional pore morphology and estimation of ionomer distribution in both Pt-based and PGM-free cathode catalyst layers (CCL) [1, 2]. On the other hand, due to insufficient resolution (20 nm voxel size) this technique cannot resolve the detailed structures of support, ionomer, and catalyst. We developed the “hybrid computational method” [2] combining statistical information from transmission electron microscopy (TEM), ultra-small angle X-ray scattering (USAXS), and BET with nano-CT to reconstruct the features below the nano-CT resolution. In this study, an example will be given of application of this method to a Pt alloy-based electrode. Various shape, size and composition agglomerates are reconstructed to 1 nm voxel resolution using the hybrid methodology. It is shown that ionomer film thickness is spatially non-uniform inside the agglomerates and strongly correlates with ionomer to carbon weight ratio (I/C) while it is independent of the agglomerate size. Compared to experimental observations, the application of capillary condensation theory to the reconstructed agglomerate structure is shown to accurately represent the relative humidity (RH) dependence of the electrochemically-active surface area (ECA). Furthermore, direct numerical simulations are performed to quantify localized transport losses. Effectiveness factor correlations are derived for use in macroscale models. The effects of RH, agglomerate structural properties, and pores in the carbon support on the local oxygen transport resistance are examined. It is demonstrated that low catalyst utilization is responsible for the experimentally-observed high local transport resistance under dry conditions and that agglomerate shape and size affect the local oxygen transport resistance only if the primary pores are poorly accessible.

Engineering PGM-Free Electrodes to Facilitate Improved Performance for the Oxygen Reduction Reaction in Polymer Electrolyte Fuel Cells

With the aiming of reducing stack cost, the investigation of platinum group metal (PGM)-free catalysts for oxygen reduction reaction (ORR) has gained a lot of interest in the recent decades,1 with the majority of focus on improved electrocatalysis via various precursors and synthesis methods. The most promising PGM-free catalysts in terms of activity are carbon-based materials doped with nitrogen and active transition metals (like Fe, Co, and Mn), obtained by high temperature pyrolysis.2 The literature regarding synthesis, characterization, and rotating disk electrode (RDE) performance of this class of PGM-free materials is abundant, with many works reporting beginning of life performance testing in PEFC. Relative to the extensive literature on electrocatalyst fabrication, a less extensive and rigorous examination of PGM-free electrode design has been performed. A number of variables like ionomer content, catalyst deposition method (e.g. spray coating, blade coating, hand painting), ink concentration, ink solvent composition and ink evaporation rate may influence PEFC performance. All of these process variables, along with the properties of the electrocatalyst itself, can alter the electrode ionomer distribution,3 resulting is significant differences in gas transport and electrode proton conductivity.3–5 In this work, we examine the impact of ionomer content, ink deposition method, and ink solvent composition on the PEFC performance of a commercially available PGM-free catalyst (Pajarito Powder LLC). Electrode properties will be assessed using different electrochemical diagnostics techniques. The viability of H2 limiting current will be discussed as a means to extract the pressure dependent and independent mass transport resistance of the electrode layer.6,7 Cyclic voltammetry and electrochemical impedance spectroscopy will be used to determine electrode capacitance and subsequently related to ionomer coverage and proton conductivity8,9 and contrasted with SEM images of MEA cross sections. Through this methodology we will demonstrate how the electrode fabrication process plays a crucial role in electrode properties and the absolute performance.

Recent Advances with Sulfonated Silica Ceramic Carbon Electrodes for Fuel Cells

Sulfonated silica-based ceramic carbon electrodes (SS-CCE) are prepared using low-cost organosilane precursors that are mixed with the carbon-supported platinum (Pt/C) catalyst in monomer form, and subsequently polymerized and coated onto a gas diffusion layer in a single step. Our lab has demonstrated that that SS-CCEs can perform very well in a fuel cell dry operating conditions, due to the proton conductivity of the sulfonic acid moiety and the hygroscopic nature of the SiO2 backbone. While these results have been promising, there are few detailed studies of SS-CCE structure-property. Likewise, their durability in operating fuel cells have yet to be proven. This paper describes our group’s recent progress towards optimization the synthesis and structure of SS-CCEs in order to enhance performance and promote a more homogenous distribution of silicate ionomer. The durability of SS-CCEs is also compared to that of conventional Nafion-based electrodes.

Electrospinning and MEA Conditioning As a Tool for Designing Optimized Electrode Structures for Fuel Cell Applications

Complimentary to materials improvements, the rational design of electrode structures and interfaces have emerged as potential routes for increasing fuel cell efficiency. Traditional MEA fabrication methods are dependent upon the deposition of uniform ink compositions that couple the integration of ionomer and catalyst. Subsequent electrode morphology then depends solely on the material properties and interactions of the catalyst, ionomer and solvents being utilized. In Pt based systems, unfavorable interactions between ionomer and Pt active sites can lead to reductions in activity and additional oxygen transport resistance. [1,2] Additionally, in Platinum Group Metal (PGM)-free electrodes, inhomogeneous coverage of larger particles results in poor catalyst utilization and reduced proton conductivity throughout the layer. Electrospinning can be used to combat such limitations. Electrospinning can produce nanofiber threads (typically ~500 nm in diameter) formed by applying an electrostatic voltage from the deposition needle to the grounded substrate.[3] Electrospun electrode development gives an opportunity for us to dictate electrode properties and interactions that are more defined and desirable, by allowing us to decouple electrode structural aspects from solution or ink-based behavior. Such advantages include: (i) the ability to produce homogeneous ionomer distributions across both electrocatalyst agglomerates and the electrode thickness, (ii) greatly improve the ionomer distribution for larger catalysts structures/agglomerates, such as MOF based PGM-free electrocatalysts and (iii) utilize multiple ionomers, each of which may be specifically tailored based on local or bulk electrode requirements. Here, we will discuss the incorporation of two classes of electrocatalysts; (i) PGM based (e.g. Pt/C and Pt alloy/C)s and (ii) and PGM-free (e.g. FeNx/C based) . The electrospun electrodes were fabricated with different ionomer (Nafion), carrier polymer, (PVDF, PAA) and solvent blends (IPA, DMF, Acetone). The effect of varying synthetic and fabrication parameters (flow rates, humidity, and operating potential) on electrode structure were investigated in order to improve high current density performance. The electrodes were characterized using various microscopic techniques (TEM, SEM, EDS), and in-situ diagnostics such as oxygen reduction reaction (ORR) mass activity, electrochemical surface area (ECA) and H2-Air/O2 performance, oxygen transport resistances (Rnf) and Electrochemical Impedance Spectroscopy (EIS) in the catalyst layers were measured to elucidate the effects of different electrode structures on PEMFC performance. Preliminary results show improvement in mass activity (determined at 0.9V, 150 kPaabs, 80oC and 100% RH) when compared to traditional ultrasonically sprayed catalyst coated membrane (CCM) electrodes. Reductions in non-Fickian transport resistances (RnF) were also observed on nanofiber electrodes, suggesting a re-arrangement in ionomer coverage and distribution on Pt-surface. The results from this study were subsequently applied to thicker (~100 μm) platinum group metal free (PGM-Free) electrodes where optimization of gas and proton transport are more impactful.

Influence of Ink Composition and Operation Parameters on the Performance of Fe-N-C

The application of proton exchange fuel cells (PEFCs) in the automotive sector is rising to be an alternative to petroleum fuel based transportation. Nevertheless, the obstacle of cost reduction of the PEFC remains, which is due to highly expensive Pt catalysts on the cathode side for oxygen reduction reaction (ORR).[1] A possible way to substitute Pt for ORR is the use of Fe-N-C-catalysts which have been shown to be a promising option.[2] For Fe-N-C-catalysts the required catalyst loading needs to be significantly higher than for Pt-based catalysts in order to achieve sufficient current densities. This leads to a change in catalyst layer thickness of roughly one order of magnitude where transport phenomena of gases and liquids become more important.[3] Based on this, the gap between Pt/C and Fe-N-C at high current densities is significantly higher compared to the kinetic controlled region. Therefore, further optimization is needed in order to enhance activity also under high current flow. In addition to this, it need to be understood to what extent problems in the transport properties affect the durability behavior of Fe-N-C catalysts in fuel cells. The ionomer distribution and loading of the catalyst layer has found to be crucial for proton transport towards the active sites, but also influences the hydrophilicity of the catalysts pores which enhances gas transport resistance. [1] In addition, it has been shown that the preparation of the membrane electrode assembly (MEA) for the fuel cell plays a key role. [3] The widely used hot-pressing technique might influence the catalyst layer pore size which is also an important parameter for species transport. In this work, different catalyst to ionomer ratios are evaluated in fuel cell tests for Fe-N-C catalysts. Furthermore, the preparation by hot-pressing is investigated in respect to the application of different compression pressures. The results are brought in relation to structural investigations of the MEAs and serve for the validation of a theoretical model. [1] S. Komini Babu, et al. ACS Appl. Mater. Infertaces 2016, 8, 32764-32777 (2016) [2] U.I. Kramm, et al. J. Am. Chem. Soc., 138 (2), 635–640 (2016) [3] X. Yin, et al. ECS Transactions, 77 (11) 1273-1281 (2017) [4] S. Komini Babu, et al. Journal of The Electrochemical Society, 164 (9), F1037-F1049 (2017)

(Invited) Materials and MEA Design Impact on the High-Current Density Performance of PEMFCs

In order to achieve platinum specific power densities of <0.1 gPt/kW, which are required for the large-scale implementation of fuel cell electric vehicles (FCEVs) powered by proton exchange membrane fuel cells (PEMFCs), both the mass activity of the catalyst for the oxygen reduction reaction (ORR) and the high-current density performance of PEMFCs must be improved [1]. The latter strongly depends on the membrane electrode assembly (MEA) materials and design, for example: i) on the morphology of the catalyst support [2]; ii) on the distribution of the Pt nanoparticles on/within the primary particles of the carbon support [3,4]; iii) on the surface functionalization of the carbon support [5]; and, iv) on the ionomer distribution in the electrode, which is affected by the solvents used for catalyst ink preparation [6]. In this contribution, we will examine the influence of Pt nanoparticle distribution, carbon support surface modification, and various catalyst ink solvents on the high-current density performance of MEAs under differential flow conditions. This will be accompanied by fuel cell diagnostics (oxygen and proton transport resistance measurements in the cathode electrode) and by ex-situ characterization methods.

Electron Microscopy Study of Degradation Mechanisms in Platinum Group Metal-Free Catalysts

Commercial low-temperature polymer electrolyte membrane fuel cells (PEMFCs) rely on expensive platinum-based catalysts to drive the oxygen reduction reaction at the cathode. In order to overcome cost barriers limiting market penetration in both transportation and stationary applications, significant efforts are being made to develop suitable platinum group metal-free (PGM-free) catalysts. The latest classes of PGM-free catalysts show very promising mass activity, but the poor durability of these carbon-based materials is still a significant challenge that must to be addressed. In this effort, advanced analytical electron microscopy methods were employed to study the degradation pathways of PGM-free catalysts which were submitted to accelerated stress tests. Catalysts provided by four different suppliers were tested concurrently in a specially designed segmented fuel cell. Polarization curves were recorded both before and after a durability stress test which involved 10k square wave cycles between 0.95 – 0.60 V in air. Analytical electron microscopy performed across a wide length scale (sub-nanometer to micron) will be presented which links the observed performance losses with changes to the atomically dispersed active sites, carbon support, and ionomer distribution. The advantages and disadvantages of different catalyst precursors (metal organic frameworks, cyanamide/polyaniline, etc.) will also be discussed from a durability standpoint. This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office under the Electrocatalysis Consortium (ElectroCat). Microscopy was performed as part of a user project at ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility

Impact of Surface Functionalization of Pt/Graphitized Carbon on Catalyst Layer Structure and PEMFC Performance

The understanding of catalyst layers (CL) structure is of great importance to reduce Pt loading in a MEA of PEM fuel cell. Recent studies have shown that the CL structure is largely dependent on ionomer microstructure, affecting Pt utilization by controlling reactant transport [1]. The ionomer microstructure such as the distribution of ionomer thin film in the CL can be controlled by introducing functional groups on the carbon support to tune the interaction between PFSA ionomers and carbon supports, which was demonstrated by MD calculation [2] as well as experimental MEA study [3]. We have tried to introduce the surface functional groups to the Pt loaded highly graphitized carbon supports using non-covalent approaches [4] to resolve the non-uniform ionomer distribution issue, particularly in highly graphitized carbon-based catalysts. The pyrene aromatic compounds with different functional groups to form different surface charge were applied to the Pt/C commercial catalyst, and the surface modified catalysts were fabricated to cathode CL of MEAs through decal process. The MEA performance evaluated in a single cell showed that the Pt/C catalyst modified with amine groups performs better than the catalyst modified with other functional groups and non-functionalized reference catalyst. The reason of better MEA performance of the surface modified Pt/C catalyst will be discussed in terms of better ionomer distribution in the CL. In addition, the issues affecting the MEA performance such as the Pt poisoning by the functional groups during the non-covalent functionalization of Pt/C will also be presented. [1] F. C. Cetinbas, R. K. Ahluwalia, N. Kariuki, V. D. Andrade, D. Fongalland, L. Smith, J. Sharman, P. Ferreira, S. Rasouli, D. J. Myers, J. Power Sources 344 (2017) 62. [2] T. Mashio, A. Ohma, T. Tokumasu, Electrochim. Acta 202 (2016) 14. [3] A. Orfanidi, P. Madkikar, H. A. El-Sayed, G. S. Harzer, T. Kratky, H. A. Gasteiger, J. Electrochem. Soc. 164 (2017) F418. [4] H.-S. Oh, H. Kim, Adv. Funct. Mater 21 (2011) 3954.

(Invited) Fundamental Studies of PEM Fuel Cell Catalyst Layer Architectures

Although fuel cells are being deployed in cars in limited commercialization, they still fall short of the DOE targets for this technology, which are required for widespread consumer acceptance. The FC-PAD (Fuel Cell Performance and Durability) consortium was formed to advance the performance and durability of polymer electrolyte membrane fuel cells (PEMFCs) at a pre-competitive level to further enable their commercialization. The primary component that FC-PAD works to understand and improve is the catalyst layer. The primary catalyst layer architecture in use is a porous dispersed Pt-supported carbon catalyst with recast solid ionomer component forming a continuous (percolating) network through the electrode for proton transport; the carbon facilitating the electrical conduction; the active electrochemical reactions conducted on the metallic Pt sites. These thin-layer-electrode films were pioneered at Los Alamos National Labs in the late 1980’s under the direction of Dr. Shimshon Gottesfeld [1,2] As ubiquitous as the thin layer electrode structure has been used, it is still an active field of research to properly understand the structure and improve the reactant transport, water removal and catalyst utilization. The fuel cell catalyst layer is a complex structure that facilitates the electrochemical conversion of hydrogen and oxygen; it provides pathways for reactant transport, and provides both electrical and proton conducting pathways. This has been traditionally referred to as the 3-phase boundary of an electrolyte, an electrode, and a gaseous fuel. In addition to the gas-reactant transport to the active site, the product water must also be managed both in terms of product removal and optimal hydration of the membrane/ionomer providing the proton conductivity. Recent FC-PAD results have examined the structure of the electrode layer. Because performance is largely influenced by the ionomer/catalyst interface within the catalyst layers of a MEA, the “ideal” interface should contain platinum particles with 100% available proton and electrical pathways to maximize catalyst utilization. In addition, the thickness of ionomer film over the catalyst nanoparticles should be optimal to facilitate gas diffusion and water balance without sacrificing its protonic conductivity. LANL’s recent FC-PAD examinations have included using neutron reflectometry to examine the interaction of ionomer (with and without cations such as cerium present as a radical scavenger) with platinum and carbon simulating the electrode structure. Other fundamental properties of the electrode layer being examined includes, the structure and size of catalyst agglomerates, the porosity and relative hydrophobicity of electrode layer pores, and relative distribution of ionomer. A combination of AFM (atomic force microscopy) BET, MIP (mercury intrusion porosimetry), IGC (inverse gas chromatograph) have been employed to help better define these catalyst layer fundamental properties. These types of measurements suggest an inhomogeneous distribution in the electrode layer. Such an inhomogeneous distribution indicates that catalyst utilization is not fully optimized. This talk will discuss many of the factors affecting catalyst layer performance and provide strategies for catalyst layer optimization. Acknowledgments This work was funded through the DOE FC-PAD Consortium with thanks to DOE EERE FCTO, Fuel Cell Team Leader: Dimitrios Papageoropoulos and Technical Development Manager: Greg Kleen. References Wilson, M. S.; Gottesfeld, S., Thin- film catalyst layers for polymer electrolyte fuel cell electrodes, Journal of Applied Electrochemistry (1992), 22(1), 1-7. Wilson, M. S.; Gottesfeld, S, High Performance Catalyzed Membranes of Ultra‐low Pt Loadings for Polymer Electrolyte Fuel Cells, Journal of the Electrochemical Society, Electrochem. Soc. 1992,

(Invited) Progress Towards High Power Density and Durable PEFCs with PGM-Free Cathodes

This presentation will highlight recent advances in achieving high power density with polymer electrolyte fuel cells (PEFCs) that use platinum-group metal-free (PGM-free) catalysts for the cathode’s oxygen reduction reaction (ORR). PGM-free catalysts have the potential to radically reduce the cost of manufacturing PEFC cathodes. However, several technical challenges with PGM-free cathodes need to be addressed, including a need for increased catalyst activity and stability as well as improved catalyst integration into MEAs for higher power density and better durability. Electrode engineering is crucial to achieving high performance with PGM-free catalysts because the lower volumetric activity yields thicker electrodes that cause significant water flooding and large oxygen and proton transport losses. This talk will highlight recent advances in the field as well as our own recent achievements with PGM-free catalysts prepared from chemically-doped metal organic framework (MOF) precursors. The focus of the talk will be on the electrode integration of these catalysts, including a systematic optimization of the catalyst morphology for MEA integration that leverages the ability to carefully tune the size of the MOF-derived catalyst’s primary particles. A key aspect of our study was the control of particle morphology for effective ionomer distribution. Through combined computational simulation, experimental fuel cell characterization, and electron and high resolution X-ray imaging, we show the significant performance improvements that can be achieved. Furthermore, we present the current status of the MEA durability and will discuss current hypotheses regarding the degradation mechanisms and possible mitigation strategies. The authors gratefully acknowledge research support from the Electrocatalysis Consortium (ElectroCat), established as part of the Energy Materials Network under the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, under Contract Number DE-EE0008076.