PEM Fuel Cell Electrodes Research Articles

(Invited) Catalysts, Inks and Electrodes for PEM Water Electrolysis: Scalable Approach for Highly Performed Catalyst Layer Fabrication

The majority of commercially available anodic electrocatalysts for Low Temperature PEM Water Electrolyzers are based on unsupported iridium-based materials. These electrocatalysts can be amorphous or crystalline metallic, oxidic, and/or hydroxide-derived compounds. The planned worldwide installations of electrolyzers (on the order of tens of Gigawatts per year) will require manufacturing of Membrane Electrode Assemblies (MEAs) via highly scalable methods, one of which is Roll-2-Roll production (R2R) [1, 2]. In contrast to spraying techniques, R2R requires the utilization of inks with a higher content of solids, which in the case of extremely dense Ir-based materials may cause a sedimentation of suspended electrocatalysts resulting in overall ink destabilization.In the presented work a systematic study of ink stability with commercial IrO2 catalysts over a period of 14 days was performed. During this period of time the minimal redispersion of slurries was done to mimic realistic conditions of inks aging. The inks were comprehensively characterized by rheological measurements, zeta potential, particle size distribution (Dynamic Light Scattering) and inks density. A series of decal electrodes was fabricated using Baker bar coating approach (a model of R2R production) and coatings were characterized by contact angle, X-Ray Photoelectron Spectroscopy and Transmission Electron Microscopy.Detailed electrochemical performance analysis including the separation of voltage losses [3] via Electrochemical Impedance Spectroscopy was performed to compare MEA performance with aging time. Acknowledgements and Statements. The authors acknowledge financial support from M2FCT and H2NEW consortia. References. [1] J. Sharma, X. Lyu, T. Reshetenko, G. Polizos, K. Livingston, J. Li, D. L. Wood, A. Serov "Catalyst layer formulations for slot-die coating of PEM fuel cell electrodes" Int. Journal of Hydrogen Energy 47 (2022) 35838-3585.[2] E. B. Creel, K. Tjiptowidjojo, J. A. Lee, K. M. Livingston, P. R. Schunk, N. S. Bell, A. Serov, D. L. Wood III "Slot-Die-Coating Operability Windows for Polymer Electrolyte Membrane Fuel Cell Cathode Catalyst Layers" J. of Colloid and Interface Science 610 (2022) 474-485.[3] A. Dizon et. al “Advanced Voltage Break Down Analysis by Statistical Open-Source Tool Case Study Based on Polymer Electrolyte Water Electrolysis”, ECS 242 (2022)

FIB‐SEM and ToF‐SIMS Analysis of High‐Temperature PEM Fuel Cell Electrodes

AbstractThe phosphoric acid (PA) distribution in the electrodes is a crucial factor for the performance of high‐temperature polymer electrolyte fuel cells (HT‐PEM FCs). Therefore, understanding and optimizing the electrolyte distribution is vital to maximizing power output and achieving low degradation. Although challenging, tracking the PA in nanometer‐sized pores is essential because most active sites in the commonly used carbon black‐supported catalysts are located in pores below 1 µm. For this study, a cell is operated at 200 mA cm−2 for 5 days. After this break‐in period, the cathode is separated from the membrane electrode assembly and subsequently investigated by cryogenic focused ion beam scanning electron microscopy (cryo FIB‐SEM) coupled with energy‐dispersive X‐ray spectroscopy (EDX) and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS). PA is located on the surface and in the bulk of the cathode catalyst layer. In addition, the PA distribution can be successfully linked to the gas diffusion electrode morphology and the binder distribution. The PA preferably invades nanometer‐sized pores and is uniformly distributed in the catalyst layer.

Catalyst layer formulations for slot-die coating of PEM fuel cell electrodes

The series of electrodes were fabricated by the scalable and manufacturable slot-die coating method for proton exchange membrane fuel cell (PEMFC) application. The inks with different amounts of solids were studied by rheological methods in order to establish a coating window with minimum manufacturing defects. The obtained electrodes were characterized by SEM, AFM, and optical microscopy, which showed that they were uniform and homogeneous with minimum defects. The electrochemical evaluation of the manufactured gas diffusion electrodes (GDE) showed that the main characteristics of the electrodes, like electrochemical surface area, proton resistivity, and double layer capacitance, were found to be close for all samples confirming the reproducibility of the slot-die process. Additionally, we studied the effects of membrane thickness on the performance of the GDE membrane electrode assemblies and determined that a decrease in membrane thickness favored the performance. The obtained results clearly demonstrated the applicability and feasibility of the approach for the Manufacturing of catalyst layers for the fuel cell application with potential for future mass production.

High-temperature PEM fuel cell electrode catalyst layers part 1: Microstructure reconstructed using FIB-SEM tomography and its calculated effective transport properties

The main topic of this study is the microstructure of catalyst layers (CLs) of a high-temperature gas diffusion electrode in a PEM fuel cell, bonded by polybenzimidazole. Using focused ion beam scanning electron microscopy in both the secondary and backscattered electron modes, Pt distribution in the CLs was studied. It was found that the choice of binder and preparation history had an important impact on the CL 3D microstructures. Polybenzimidazole-bonded CLs exhibited smooth surface, low porosity and homogeneous distribution of Pt. On the other hand, penetration of Pt into the microporous layer of the electrode was observed. Good thermal and mechanical stability of the CL made it possible to carry out FIB-SEM tomography and to reproduce its 3D microstructure by alternating use of ion milling and imaging of emerging cross-sectional planes. The 3D microstructure obtained served as an input for mathematical models allowing an evaluation of the effective macroscopic transport properties of the CL. This represents important information for reliable mathematical modelling and optimisation of the high-temperature PEM fuel cell.

High-temperature PEM fuel cell electrode catalyst layers Part 2: Experimental validation of its effective transport properties

In this study, the experimental approach for determination of high-temperature PEM fuel cell, PBI-bonded catalyst layer conductivity and transfer properties was developed and optimised. Electrical conductivity was determined by chronopotentiometry combined with mathematical model, using uncompressed catalyst layer on a conductive glass support. Effective transport properties were determined using gas-diffusion electrode samples fixed in the Wicke-Kallenbach cell. Experimental results were compared with quantities calculated from FIB-SEM tomography. Very good agreement between the results was reached. Theoretical data evaluated on the base of tomography may thus be considered as validated. The results confirm the heat treatment of the catalyst layer to lead to its reorganisation to less porous and better-defined structure with improved electrical conductivity. Changes in effective transport properties, however, were not pronounced. This is due to the nature of gas-diffusion electrode samples. Determined electrical conductivities and transfer properties of catalyst layer underlined the importance of gas-diffusion electrode preparation procedure. This represents an important input for further studies, focused on optimisation of membrane-electrode assemblies for HT PEM FC.

Catalysts Containing Transition Metals on Nitrogen and Carbon Structures for Hydrogen Production through Salt Water Electrolysis

Hydrogen as an energy carrier is promising as it does not produce green-house gas (GHG) emissions at points of use and can be obtained from sources of low, or even neutral or negative GHG intensities [1,2]. One pathway to produce low emission hydrogen is water electrolysis when using a renewable power source [3]. However, freshwater, commonly used in this process, is relatively scarce, demand is high and expected to rise, and gets scarcer every day because of pollution [4,5,6]. An alternative is using saltwater, either through desalination or directly via adequate electrolysers [4]. Regarding direct use, a major challenge concerns catalyst, typically made of precious metals like Pt in the cathode, which are expensive, limited and susceptible to poisoning, e.g., by chlorine [7]. Likewise, in the anode chlorine is corrosive and reduces faradaic efficiency [4]. In this context, metal-coordinated nitrogen-carbon (M-N-C) nanostructures have proven to be attractive vis-à-vis these challenges [8]. To study their potential for saltwater electrolysis, five materials were synthesized following Malko et al. [8] using 1,5-diaminonaphthalene and 1% wt metallic sulphate salts of Co, Ni, Fe, Ni with Fe, and a blank without metal. The resulting material were suspended in 2-propanol and cast onto a vitreous carbon rotating ring disk electrode in both acidic solution 0,5M H2SO4 and saline medium 0,5M NaCl. Following electrode conditioning with cyclic voltammetries and electrochemical impedance characterization, linear sweep voltammetries (LSVs) were carried out in order to assess activity towards hydrogen evolution reaction (HER), coupled to hydrogen oxidation (HOR) at the Pt electrode in a ring-RDE (RRDE) fashion at constant potential of 0,3 V.The image presents HER and HOR plots in RRDE experiments at 1800 rpm, using either N-C or NiFe-N-C at a load of 0,27 mg cm-2 of catalyst at the disk and a Pt ring: (top) LSV at the disk (current normalized by geometric area, v = 2 mV/s), and (bottom) polarization at the Pt ring (E=0.3V vs NHE). Shown are both acidic medium (0.5 M H2SO4, N-C in 1 blue and NiFe-N-C in 2 red) and saline medium (0,5 M NaCl, N-C in 3 cyan and NiFe-N-C in 4 magenta). It was found that NiFe-N-C catalyst is active towards HER in saline medium at near neutral pH. As anticipated, the activity in acidic media is significantly higher than on saline medium, attributing this difference to several aspects such as conductivity limitations, chlorine presence and absence of free H+ ions and the need of additional steps for water dissociation [9]. In acidic media the metal-free catalyst (N-C) shows larger currents for most of the potential widow. On the other hand, the ring electrode does not show activity at all for N-C, thus it may be concluded that there is no H2 evolution and therefore the current is mostly capacitive, this capacitive contribution will be subtracted from the final results. In contrast, in saline medium N-C shows no activity at all towards HER, even showing smaller capacitive currents, while NiFe-C shows smaller albeit significant currents given the large differences in pH between media. The metal catalysts, the blank and a Pt/C in acidic and saline medium, will be compared using onset potentials, overpotentials at 10 mA/cm2 and Tafel slopes. References Blagojević , V. Minić, D. Minić D. & Grbović Novaković J. Hydrogen Economy: Modern Concepts, Challenges and Perspectives, Hydrogen Energy, IntechOpen (2012).Zhang,Y. & Zou, X. Noble metal-free hydrogen evolution catalysts for water splitting, Chemical Society Reviews, 15, (2015).IEA webstore, The Future of Hydrogen Seizing today´s opportunities, (2019).Dresp, S. Dionigi, F. Klingenhof, M. & Strasser, P. Direct Electrolytic Splitting of Seawater: Opportunities and Challenges, ACS Energy Letters (2019).Boretti, A. y Rosa L. Reassessing the projections of the World Water Development Report, npj Clean Water, 2, 15, (2019).Vos, J. & Koper, M. Measurement of competition between oxygen evolution and chlorine evolution using rotating ring-disk electrode voltammetry, Journal of Electroanalytical Chemistry, 819, (2018).Sethuraman, V. A. & .Weidner, J. W. Analysis of Sulfur Poisoning on a PEM Fuel Cell Electrode, Electrochimical acta, 55, 20, (2010).Malko, D. Lopes, T. Symianakis§a E. & Kucernak, A. The intriguing poison tolerance of non-precious metal oxygen reduction reaction (ORR) catalysts, Mater. Chem. A, 4, (2016).Zhou, Z. Pei, Z. Wei, L. Zhao, S. Jian, X. & Chen, Y. Electrocatalytic Hydrogen Evolution under Neutral pH Conditions: Current Understandings, Recent Advances, and Future Prospects, Energy Environ. Sci., 10, (2020) Figure 1

Adaptation and Validation of a Pore-Network Model for Tracking Water in PEM Fuel Cell Electrodes

Liquid water in PEM fuel cell porous transport layers, also known as gas diffusion layers, has been extensively studied in order to understand how to remove product water and reduce mass trasnport losses. A variety of pore network models and imaging techniques have been developed and successfully used to model water transport in these porous layers. Mass transport losses also occur in the electrodes (catalyst layers) due to water where there is little understanding of how water interacts with the ionomer films and percolates through the pore space. In order to better understand liquid water movement, holdup, and evaporation in catalyst layers a pore network model has been adapted for use on PEM fuel cell electrodes. Network models are built on an abstraction of geometric features. Therefore, ex-situ characterization tests are needed to determine if the abstraction is valid. This talk focuses on the electrode network model development, characterization, and parametric studies examining the interplay between production of heat and water with transport through the catalyst support, pore space and ionomer films.

Structured Electrodes for PEM Fuel Cells

Slow reaction rates for oxygen reduction reaction (ORR) is one of the major barriers for improving PEMFC performance. Optimizing electrode structures for enhanced transport properties can improve high current density performance of PEMFCs. Conventional porous electrodes are fabricated by coating or spraying techniques with minimal control of its micro-structure, rendering random and tortuous transport pathways (Fig. 1a left). As a result, proton transport through the ionomer and oxygen diffusion in primary and secondary pores are sub-optimal and often become a performance limiting factor depending on the cell’s operation conditions. Therefore, a carefully designed electrode micro-structure should significantly improve proton and oxygen transport. In this work we developed structured electrodes (Fig. 1a right) for Platinum based PEMFCs where different components of the electrode serve a dedicated function (i.e., transport of proton or oxygen). These electrodes are highly ordered to reduce transport losses associated with tortuosity and non-percolation. In addition, the structural attributes are controlled on multiple length scales to minimize transport resistance and improve the PEMFC performance. For design optimization and to improve the fundamental understanding of the transport mechanism in the electrode, we developed a state-of-the-art multi-physics electrode model to perform parametric study. The model accurately accounts for ORR kinetics, agglomerate structure, proton transport in bulk membrane and thin film ionomer, oxygen diffusion in the pores, heat transfer, as well as vapor and liquid phase water transport in the electrode. By varying geometric and cell operating conditions, we conducted more than 12,000 simulations using COMSOL Multiphysics to identify critical electrode design parameters that enable fast reaction and improved transport phenomena. Results from our continuum scale simulation show significant performance improvement for structured electrodes compared to conventional electrodes (Fig. 1b). The performance improvement is due to the reduced proton and oxygen transport resistances. Altogether, our findings unveil structured electrodes as a new resource for efficient charge and mass transport, and as a promising material design platform towards obtaining reliable PEMFCs with enhanced energy conversion performance. Figure 1

Effect of Carbon Corrosion on Wettability of PEM Fuel Cell Electrodes

Large transport resistances at high current densities hinder the proton-exchange-membrane fuel cells from reaching performance-cost-durability targets. Extensive carbon corrosion in the cathode catalyst layers has been shown to correlate to pore closures and increase in surface oxides of carbon support, which has been associated with the increase in wettability of the catalyst layers. This study investigated effects of changes in microstructure and surface oxides on the change of wetting properties of cathode catalyst layers after different accelerated stress tests. The apparent contact angles were measured over a broad range of cathode carbon mass losses at 0.3 mgPt/cm2. The apparent contact angles fell sharply after small extent of carbon loss followed by a plateau, and decreased again after 123 µg/cm2 (35% carbon support loss in the cathode). The microstructural changes were characterized by surface roughness (Atomic Force Microscopy) and porosity (focus-ion-beam scanning electron microscopy and thermogravimetric analysis). The surface roughness and porosity were used to parameterize Wenzel’s and Cassie-Baxter’s models, respectively, to elucidate the changes in Young’s contact angle. The fittings of the two models revealed that the cathode catalyst layers sustained their wettability after up to 35% of carbon support loss due to the increases in the proportion of hydrophobic fluorocarbon chains on the surface of the catalyst layer from XPS. The results suggest that the mass-transfer resistance cannot be ascribed to greater liquid saturation of the pore structure before the cell significantly fell below the DOE’s technical target.

Optimal Regioselectivity Control in Electrocatalytic Oxidation of Terminal Olefins

Wacker-type reaction has been considered as an effective industrially viable route for oxidation of various organic compounds since 1970s [1]. Products of these processes, such as aldehydes and ketones are extremely valuable in fine chemicals industry [2]. Despite the historical success of these reactions, there hasn’t been many studies in terms improving their processes throughput, and particularly the attributed Palladium catalyst efficiency. During the past decade, oxidation of organic compounds via electrocatalytic oxidation has provided a valuable opportunity to increase the reaction efficiency as well as tuning the metallic catalyst specifically designed for these processes [3]. Electrochemistry is a well-known method used for a multitude of purposes ranging from renewable energy production [4], corrosion protection [5], and organic syntheses [3]. This method has been used in large scale production of different metal oxides, commodity chemicals and active pharmaceutical ingredients (e.g. acetone, and cephalosporin, respectively) [6]. However, the true potential of this configuration for organic synthesis is yet to be discovered. The recent reports on electrochemical Wacker oxidation of olefins as model organic substrates has shown that through this process modification, a higher yield compared to traditional batch configuration can be achieved [3]. Nonetheless, the effect of catalyst tuning and property optimization on electrochemical Wacker oxidation is a critical component towards process intensification. Solid-supported catalysis has been found to be one of the most successful methods to improve upon traditional homogeneously catalyzed reactions [7]. During this catalyst-preparation method, metallic particles of catalyst are immobilized on the surface of a support capable of high charge transfer (e.g. graphene [8]). Hence, the redox reactions occurring during the catalytic cycle of the metal particles are improved by faster and more efficient electrical charge transfer through the support. One of the major drawbacks of the electrochemical approach for Wacker-type reactions is the lack of control over regioselectivity of olefin oxidation. Therefore, a mixture of two different products (aldehyde and ketone) (Figure 1) is often obtained. It has been shown that by promoting an anodic oxidation or cathodic reduction of catalyst, a certain regioselectivity can be promoted [9]. Thus, the type of product (aldehyde or ketone) can be controlled. Figure 1 Regioselective Wacker-type electrocatalytic oxidation of terminal olefins to ketone (top) and aldehyde (bottom) In this study, a palladium on graphene nanoplatelet solid-supported catalyst was prepared via an all-aqueous ascorbic acid-mediated chemical reduction process for Wacker-type electrochemical oxidation of 1-octene as a model substrate. During the process optimization turn over frequency (TOF) of the prepared electrocatalyst compared to conventional homogeneous PdCl­2 [9] and solid-supported Pd on carbon nanotube [3] catalysts was improved by approximately 110 and 50 time fold, respectively. Nevertheless, through controlling the promoted half-cell reaction, oxidation or reduction, the regioselectivity of the reaction was controlled where aldehyde and ketone products were successfully prepared with optimal selectivity. Higher activity of prepared catalysts due to improved charge transfer via graphene as well as improved control over anodic oxidation through cell-type alternation are thought to be the governing factors in such improved yield and regioselectivity. References Michel, B.W., L.D. Steffens, and M.S. Sigman, The Wacker Oxidation. 2014, Wiley.Calter, M., Palladium Reagents and Catalysts: Innovations in Organic Synthesis By Jiro Tsuji (Okayama University of Science). John Wiley and Sons, New York, NY. 1995. xiv + 560 pp. 15 × 22.5 cm. ISBN 0-471-95483-7. Journal of Natural Products, 1996. 59(12): p. 1215-1215.Donck, S., et al., Tsuji–Wacker Oxidation of Terminal Olefins using a Palladium–Carbon Nanotube Nanohybrid. ChemCatChem, 2015. 7(15): p. 2318-2322.Ghobadi, S., et al., Green Composite Papers via Use of Natural Binders and Graphene for PEM Fuel Cell Electrodes. ACS Sustainable Chemistry & Engineering, 2017. 5(9): p. 8407-8415.Zhang, H., et al., Simultaneously Harvesting Thermal and Mechanical Energies based on Flexible Hybrid Nanogenerator for Self-Powered Cathodic Protection. ACS Applied Materials & Interfaces, 2015. 7(51): p. 28142-28147.Cardoso, D.S.P., et al., Organic Electrosynthesis: From Laboratorial Practice to Industrial Applications. Organic Process Research & Development, 2017. 21(9): p. 1213-1226.Yang, Y., et al., Three dimensional composites of graphene as supports in Pd-catalyzed synthetic applications. Reaction Chemistry & Engineering, 2018.Siamaki, A.R., et al., Microwave-assisted synthesis of palladium nanoparticles supported on graphene: A highly active and recyclable catalyst for carbon–carbon cross-coupling reactions. Journal of Catalysis, 2011. 279(1): p. 1-11.Mitsudo, K., et al., Electrochemical Generation of Cationic Pd Catalysts and Application to Pd/TEMPO Double-Mediatory Electrooxidative Wacker-Type Reactions. Journal of the American Chemical Society, 2007. 129(8): p. 2246-2247. Figure 1

Effect of Carbon Corrosion on Wettability of PEM Fuel Cell Electrodes

Large transport resistances at high current densities hinder the proton-exchange-membrane fuel cells from reaching performance-cost-durability targets. Extensive carbon corrosion in the cathode catalyst layers (CCL) has been shown to correlate to pore closures and increase in surface oxides of carbon support, which has been associated with the increase in wettability of CCLs. This study investigated effects of microstructure and surface oxides on the change of wetting properties of CCLs after different accelerated stress tests. The apparent contact angles were measured over a broad range of carbon mass losses at 0.3 mgPt/cm2. The apparent contact angles fell sharply after small extent of carbon loss followed by a plateau, and decreased again after 123 μg/cm2 (35% carbon support loss). The microstructural changes were characterized by surface roughness (atomic force microscopy) and porosity (focus-ion-beam scanning electron microscopy and thermogravimetric analysis). The surface roughness and porosity were used to parameterize Wenzel's, Cassie-Baxter's, and combined models to elucidate the changes in Young's contact angle. The fittings of the models revealed that the CCLs sustained their wettability after up to 35% of carbon support loss due to the increases in the proportion of hydrophobic fluorocarbon chains on the surface of the CCLs measured from XPS.

Fundamental Insights in Degradation Mechanisms of Pt/C Nanoparticles for the ORR

Despite large efforts to develop and optimize PEM fuel cell electrodes for commercialization, degradation processes on the widely used Pt/C catalyst materials are not fully understood to date. In this work, we analyzed the changes of morphology and structure of commercially available Pt/C catalysts by applying different electrochemical accelerated degradation protocols using a rotating disk electrode (RDE) technique. The Pt nanoparticles (Pt NP) as well as carbon support material were characterized by using ex situ transmission electron microscopy (TEM) and Raman spectroscopy. We point out that the growth of the Pt NP and the oxidation of the carbon support material are strongly influenced by the applied potential range. In particular, the “start/stop” experiments (2,000 potential cycles, 0.5 V - 1.5 V vs. RHE) show the highest loss in ECSA and strongest growth of Pt NP compared to the “carbon corrosion” (2,000 cycles, 1.0 V - 1.5 V vs. RHE) and “lifetime” (10,000 cycles, 0.5 V - 1.0 V vs. RHE) protocols. In addition, the Raman spectra of the treated Pt/C indicate that the electrochemical oxidation of defect-rich carbon sites is accelerated by applying higher anodic potentials (above 1.0 vs. RHE) and in the presence of smaller Pt NP. Altogether, our results provide a deeper understanding in the development of more stable and highly active PEM fuel cell catalysts.

Polyacrylic acid-Nafion composites as stable catalyst support in PEM fuel cell electrodes

A polyacrylic acid (PAA) – Nafion composite was studied as a possible catalyst support for proton exchange membrane fuel cell electrodes. PAA-Nafion is added to form a thin layer covering the freshly deposited Pt nanoparticles. For this purpose, a UV light induced photochemical reaction of the Pt precursor is carried out in the presence of a small amount of Nafion as surfactant for the reaction. Carbon is added in a later stage to enhance electrical conductivity. PAA-Nafion, being a stable polymer compound with strong binding properties, separates the Pt nanoparticles from the carbon surface, thus reducing Pt enhanced carbon corrosion. Potential cycling up to 1000 cycles in aqueous solution led to no significant change in crystallite sizes of Pt. This was also confirmed by accelerated degradation studies in a fuel cell test-bench wherein 52% of the electrochemically active surface area (ECSA) was retained after 5000 cycles, whereas a state-of-the-art commercial catalyst lost most of its activity in the first 2000 cycles.

Green Composite Papers via Use of Natural Binders and Graphene for PEM Fuel Cell Electrodes

Herein a new fabrication method is suggested via a paper molding process for the preparation of graphene/cellulose/carbon fiber composite paper to be used as the flexible electrode layers mainly for PEM fuel cells. As a green and eco-friendly process, composite papers were prepared at ease with no pollutant by product formation by using ionic liquids (IL). At first, carbon fibers bound via cellulose fiber to form a cellulose/carbon fiber composite paper by wet-laying. Then, electrical conductivity as well as hydrophobicity of the composite papers were improved by extraction of cellulose via an ionic liquid-assisted phase inversion method and then graphene inserted into the structure of ionic liquid treated composite paper by spray coating. Moreover, by spray coating of 0.5 mg cm–2 platinum (Pt) catalyst to the mentioned composite papers, significant improvement in their fuel cell performances were achieved at low relative humidity (RH) fuel cell operation conditions (RH < 50%) which increases the overall system efficiency. The highest power density was recorded at 50% RH as 911 mW cm–2, which was over 20% higher than the 100% RH operating conditions.

(Invited) Material Degradation in PEM Fuel Cell Electrodes

FC-PAD (Fuel Cell – Performance and Durability) consortium coordinates U.S. national laboratory activities related to PEM fuel cell performance and durability. The cost and durability of Polymer Electrolyte Membrane Fuel Cells (PEMFCs) are two major barriers to the commercialization of these systems for transportation power applications. Transportation conditions include operation in the presence of fuel and air impurities, start/stop, freeze/thaw, and humidity and load cycling that result in mechanical and chemical stresses. MEA durability decreases with decreasing catalyst loading, making cost reduction even more difficult. While there has been significant progress in improving PEM Fuel Cell durability with lower cost materials, further improvements are needed to meet the commercialization targets. Degradation in the cathode catalyst layer is the primary cause of performance loss, with both recoverable and irreversible losses occurring. One recoverable loss for PEMFCs includes catalyst poisoning by membrane degradation products [1] which requires removal from the catalyst surface and then from the electrode layer for the performance recovery[2]. Irreversible degradation modes include catalyst dissolution and ripening, loss of alloying agents from Pt-X catalysts, plus the effects of the various forms of carbon used in PEMFC components which include changing hydrophobicity, carbon corrosion and loss of porosity of electrode layers and Gas Diffusion Layers (GDLs). These increasing losses are primarily in the cathode catalyst layer and are attributed to both increasing transport and kinetic losses. The FC-PAD consortium is examining these degradation mechanisms to help develop improved materials and operating strategies. Corrosion of the carbon electrocatalyst support has been measured during drive-cycle operating conditions and increases with increase potential cycling from 0.4 to 0.9 V. Carbon corrosion is one of the major contributors to degradation which leads to changes in the catalyst layer structure and reduces its activity. Reduction in catalyst layer thickness is observed during operation, exacerbated during drive cycles. This reduction can be due to the loss of carbon through carbon corrosion or due to compaction; both effects likely lead to a loss of void volume. Membrane additives which increase membrane life-times, have been measured to migrate into the catalyst layer and appear to be associated with the carbon in the catalyst layers. Low potentials (0.2V) appear to be required to remove membrane fragment adsorbates which decrease catalyst activity. Pt alloy catalysts lose most of their alloying agents during operation; the alloying agents migrated throughout the ionomer. Durability implications of using Pt-X alloy catalysts will be discussed. Results related to the mentioned degradation mechanisms will be presented including characterization from TEM, SEM, XRF, XRD and electrochemical testing. Acknowledgments This work was funded through the DOE FC-PAD Consortium with thanks to DOE EERE FCTO, Fuel Cell Team Leader: Dimitrios Papageoropoulos

PEM fuel cell electrode preparation using oxygen plasma treated graphene related material serving as catalyst support for platinum nanoparticles

This work deals with the preparation and investigation of polymer electrolyte membrane fuel cell (PEMFC) electrodes, which are obtained using gas diffusion layers coated with graphene related material (GRM) serving as a catalyst support for platinum nanoparticles. PEMFC electrocatalysts have been prepared by pulsed electrochemical deposition of platinum particles from hexachloroplatinic acid. Prior to GRM decoration with platinum, the graphene structures are functionalized by oxygen plasma treatment. This leads to oxygen containing functional groups on the GRM outer surface, providing an improved hydrophilic behavior, thus favoring the Pt deposition process. Membrane electrode assemblies (MEAs) with the so prepared electrodes are investigated in-situ in our fuel cell test system. Polarization plots (in-situ cell performance) using these MEAs have been tested under different operational conditions.

The Effects of Ethanol Dehydrogenation By-Products on a PEM Fuel Cell Performance: A Multi-Technique Analysis

The catalytic dehydrogenation of ethanol on a Cu / ZrO2 catalyst at 250 ° C has been recently proposed as a suitable approach for the production of CO-free hydrogen[1]. In this way, the hydrogen-rich gas stream generated in this system had been directed to the anode of a PEM fuel cell, that presented 70% of the efficiency obtained when pure hydrogen was used. The reason for this considerable loss in performance was not clear and this is the main motivation of this work. For this investigation, we have used a system in which hydrogen was contaminated with the by-products of the ethanol dehydrogenation reaction, which were unreacted ethanol, acetaldehyde and ethyl acetate. The influence of these last substances on the performance of the PEMFC was studied through various techniques, including electrochemical impedance spectroscopy, cyclic voltammetry, linear sweep voltammetry, cell polarization response, high performance liquid chromatography (HPLC) and mass spectrometry. The results had confirmed that the impurities coming from the ethanol dehydrogenation reactor are the responsible for the loss of the fuel cell efficiency. Polarization curves showed that the presence of unreacted ethanol in the system is the main cause of this effect, wherein the acetaldehyde and ethyl acetate have smaller contribution. Studies involving online mass spectrometry indicated that there is no formation of CO or CO2 as by-products, as detected in the anode outlet of the cell. Analysis of the liquid effluent of the anode by HPLC, at several cell potentials ranging from 900 to 100 mV, revealed the presence of acetic acid as oxidation product on the PEM fuel cell electrode, with contents particularly high at the higher cell potentials (closer to 900mV). These studies showed that ethanol is the major contaminant of the electrode, either when directly administrated to the cell, or when it is formed by degradation of other contaminants at the PEMFC anode. Electrochemical impedance spectroscopy was utilized to evaluate the influence of the above-mentioned contaminants on the oxygen reduction reaction, since occurrence of their crossover from the anode to the cathode of the MEA was detected. The influence of each contaminant in the PEMFC performance was analysed in operando at low overpotentials (low currents - 72 mA to 500 mA). It can be inferred that at high potentials, the losses in the PEMFC performance is due to the poisoning of the oxygen electrode, when both anode and cathode catalyst were Pt/C 30 wt.%. From these data, it is shown that the proton transport resistance (membrane and cathode catalyst layer) and electrical contacts are not affected by the contaminants at these potentials. Studies on the increase of the Pt load on the anode, as well on the replacement of the Pt/C catalyst on the cathode by bimetallic Pt-based catalysts, such as PtCo and PtCr, proved ways to minimize the effects of the contaminants on the oxygen cathode. [1] A. G. Sato, G. C. D., Silva, V. A. Paganin, E. A. Ticianelli. ECS Transactions, 64 (2014)999-1005.

Electrochemical Noise Measurement and Analysis of Hydrogen-air PEM Fuel Cell

The method of measurement and analysis of electrochemical noise was used to test a hydrogen-air polymer electrolyte fuel cell and its electro catalytic electrodes. The measurement was carried out during stepped electrochemical aging. After each aging step, the electrochemical noise was measured and analyzed. Besides, power characteristics of the fuel cell and its impedance spectrums were collected to characterize fuel cell at different stages of aging. These data were compared with the electrochemical noise analysis results. Experimental study of electrochemical noise of PEM fuel cell electrodes in a liquid electrolyte during artificial degradation in solutions saturated by various gases was carried out. A decrease in activity due to the degradation of the electrodes was showed to be accompanied by a decrease in the integral of noise amplitude and a significantly lower dependence of frequency exponent (and associated Hurst exponent). At the first stage of noise analysis, a baseline was subtracted from the initial signal. Various approaches for baseline determination were used, namely polynomial approximation and the method of moving average. At the next stage of noise analysis, a Fourier transform was applied. A decrease in magnitude of peaks for some characteristic frequencies was shown to correlate with the decrease in load currents in the course of fuel cell aging.

The surface coverage of PEM fuel cell electrodes

A quantitative study was conducted on Hydrogen (H2), water (H2O) and carbon monoxide (CO) molecule concentration on a Pt–Ru catalyst surface used for PEM fuel cells. Strongly bonded hydrogen molecules which can withstand 100 ppm CO exposure at 50 °C were calculated as 54% of the total hydrogen bonds present on catalyst surface during pure H2 purge. The very weakly bonded hydrogen molecules that could be removed with pure inert gasses were not taken into account. Additionally, the water and hydrogen molecule content on the surface of the catalyst layer was compared and bonded hydrogen molecule ratios of 33–63% were obtained for different temperatures (25 °C, 50 °C and 75 °C) and relative humidity values (Dew Point Temperatures of 25 °C, 35 °C and 45 °C). Furthermore, 10 ppm CO exposure time at dry conditions was varied from 30 s to 1 h at 50 °C. The majority of the weakly adsorbed hydrogen molecules desorbed in the first 10 min with 300 sccm gas flow over Pt–Ru catalyst layer. The desorption process stopped after 10 min. The hydrogen molecules were bonded strong enough to stick to the surface under 10 ppm CO exposure at 50 °C. The bonded hydrogen molecule ratio of 66–67% was calculated for 10–60 min of 10 ppm CO exposure.

Improved Pt-utilization efficiency of low Pt-loading PEM fuel cell electrodes using direct membrane deposition

Direct membrane deposition was used to produce record platinum catalyst utilization efficiency polymer electrolyte membrane fuel cells. The novel membrane fabrication technique was applied to gas diffusion electrodes with low Pt-loadings of 0.102 and 0.029 mg/cm2. Under oxygen atmosphere and 300 kPaabs total pressure, 88 kW/gPt cathodic catalyst utilization efficiency with a symmetrical Pt-loading of 0.029 mg/cm2 on the anode and cathode side was achieved. This is 2.3 times higher than the Pt-utilization efficiency of a reference fuel cell prepared using a commercial Nafion N-211 membrane and identical catalyst layers, emphasizing that the improvement is purely attributable to the novel membrane fabrication technique. This value represents the highest Pt-utilization efficiency reported in literature. The results strongly motivate the application of employing direct membrane deposition techniques to prepare low resistance polymer electrolyte thin films in order to compensate for kinetic losses introduced when using low catalyst loadings.