Analysis for the Effect of Inverter Ripple Current on Fuel Cell Operating Condition

When fuel cell electrodes are prepared by a conventional spraying or slot-die coating method, there is little or no control over the multi-scale organization of catalyst particles and polymer binder. Electrode structural features such as particle and binder interconnectivity, macro-porosity, and micro-porosity are difficult to control, but are critically important when high-performance nanocatalysts are used. Nanofiber electrospinning is an electrode fabrication technique that can address fuel cell electrode morphology issues for next-generation MEAs. As a commercial fabrication method, electrospinning has been shown to be scalable, robust, and cost-effective, especially for the creation of non-woven mats of sub-micron-diameter polymer fibers. Electrospinning can also be used to prepare particle/polymer fiber networks with intra- and inter-fiber porosity. Such fiber mats have been used as the electrode material in Li-ion batteries, H2/Br2 redox flow batteries, and H2/air proton-exchange membrane fuel cells [1-4].Recent work on nanofiber electrodes at Vanderbilt has focused on inks containing pre-formed electrospun catalyst/binder fibers. With such inks, nanofiber electrode MEAs can be made using commercial roll-to-roll fabrication lines. In this talk, experimental results will be presented on nanofiber mat electrode MEAs with Pt/C catalyst and Nafion™ binder. Procedures for preparing inks with nanofibers and then using the inks to create CCMs or GDEs will be presented. Fuel cell performance and durability results with the resulting MEAs will be contrasted with fuel cell data where the electrodes are made directly from electrospun mats, i.e., by the direct transfer of a mat from the electrospinner to a GDL or membrane. The effects of catalyst loading and fuel cell operating conditions (backpressure, feed gas flow rate, and feed gas relative humidity) on fuel cell power output before and after metal dissolution and carbon corrosion accelerated stress tests will also be discussed. As an example of nanofiber electrode performance: an MEA with a Nafion 211 membrane and TKK Pt/C anode and cathode catalyst (0.1 mg/cm2 anode and 0.22 mg/cm2 cathode) operating at 100% RH, 80oC, and 3 atm. total pressure generated 1.0 W/cm2 at 0.7 V. Acknowledgment This work was funded by the U.S. Department of Energy’s Hydrogen and Fuel Cell Technologies Office, for the Fuel Cell Performance and Durability Project, contract No. DE-EE0007653. References C. Self, M. Naguib, R. E. Ruther, E. C. McRen, R. Wycisk,G. Liu, J. Nanda, and P. N. Pintauro, “High Areal Capacity Si/LiCoO2 Batteries from Electrospun Composite Fiber Mats”, ChemSusChem, 10, 1823 – 1831 (2017).Saadi, X. Fan, S. S. Hardisty, P. Pintauro, and D. Zitoun, “Ultralow Platinum Loading for Redox-Flow Battery by Electrospinning the Electrocatalyst and the Ionomer in Core-Shell Fibers”, Journal of Energy Storage, 59, 106430-106435 (2023).Slack, M. Brodt, D. A. Cullen, K. S. Reeves, K. L. More, and P. N. Pintauro, “Impact of Polyvinylidene Fluoride on Nanofiber Cathode Structure and Durability in Proton Exchange Membrane Fuel Cells”, Journal of the Electrochemical Society, 167, 054517 (2020).Waldrop, J. J. Slack, C. Gumeci, J. Parrondo, N. Dale, K. Shawn Reeves, D. A. Cullen, K. L. More, and P. N. Pintauro, “Electrospun Nanofiber Electrodes for High and Low Humidity PEMFC Operation”, Journal of the Electrochemical Society, 170, 024507 (2023).

When fuel cell electrodes are prepared by a conventional spraying or slot-die coating method, there is little or no control over the multi-scale organization of catalyst particles and polymer binder. Electrode structural features such as particle and binder interconnectivity, macro-porosity, and micro-porosity are difficult to control but are critically important when high-performance nanocatalysts are used. Nanofiber electrospinning is an electrode fabrication technique that can address fuel cell electrode morphology issues for next-generation MEAs. As a commercial fabrication method, electrospinning has been shown to be scalable, robust, and cost-effective, especially for the creation of non-woven mats of sub-micron-diameter polymer fibers. Electrospinning can also be used to prepare particle/polymer fiber networks with intra- and inter-fiber porosity. Such fiber mats have been used as the electrode material in Li-ion batteries, H2/Br2 redox flow batteries, and H2/air proton-exchange membrane fuel cells [1-4].Recent work on nanofiber electrodes at Vanderbilt has focused on inks containing pre-formed electrospun catalyst/binder fibers. With such inks, nanofiber electrode MEAs can be made using commercial roll-to-roll fabrication lines. For this poster presentation, experimental results will be presented on nanofiber mat electrode MEAs with Pt/C catalyst and Nafion™ binder. Procedures for preparing inks with nanofibers and then using the inks to create CCMs or GDEs will be shown. Fuel cell performance and durability results with the resulting MEAs will be contrasted with fuel cell data where the electrodes are made directly from electrospun mats, i.e., by the direct transfer of a mat from the electrospinner to a GDL or membrane, and made using a conventional catalyst powder + binder ink. The effects of catalyst loading and fuel cell operating conditions on fuel cell power output before and after a metal dissolution accelerated stress tests will also be presented. For example, an MEA with a Nafion 211 membrane and electrodes made using an ink with pre-formed fibers (TKK Pt/C catalyst at 0.13 mg/cm2 for the anode and 0.20 mg/cm2 for the cathode) generated 0.99 W/cm2 at 0.7 V, at 100% RH, 80oC, and 3 atmabs total pressure with gas feeds of H2 and air.AcknowledgmentThis work was funded by the U.S. Department of Energy’s Hydrogen and Fuel Cell Technologies Office, Fuel Cell Performance and Durability Project, under contract No. DE-EE0007653.References C. Self, M. Naguib, R. E. Ruther, E. C. McRen, R. Wycisk,G. Liu, J. Nanda, and P. N. Pintauro, “High Areal Capacity Si/LiCoO2 Batteries from Electrospun Composite Fiber Mats”, ChemSusChem, 10, 1823 – 1831 (2017).Saadi, X. Fan, S. S. Hardisty, P. Pintauro, and D. Zitoun, “Ultralow Platinum Loading for Redox-Flow Battery by Electrospinning the Electrocatalyst and the Ionomer in Core-Shell Fibers”, Journal of Energy Storage, 59, 106430-106435 (2023).Slack, M. Brodt, D. A. Cullen, K. S. Reeves, K. L. More, and P. N. Pintauro, “Impact of Polyvinylidene Fluoride on Nanofiber Cathode Structure and Durability in Proton Exchange Membrane Fuel Cells”, Journal of the Electrochemical Society, 167, 054517 (2020).Waldrop, J. J. Slack, C. Gumeci, J. Parrondo, N. Dale, K. Shawn Reeves, D. A. Cullen, K. L. More, and P. N. Pintauro, “Electrospun Nanofiber Electrodes for High and Low Humidity PEMFC Operation”, Journal of the Electrochemical Society, 170, 024507 (2023).

The lifetime of proton exchange membrane (PEM) fuel cells depends largely on the durability of the PEM and is a critical issue in the commercialization of PEM fuel cell technology (1, 2). Perfluorosulfonated acid (PFSA) membranes, such as Nafion®, are the most widely used electrolyte material for PEM fuel cells.Many factors can cause PEM failure, including manufacturing and design issues, material properties, and fuel cell operation conditions. Each factor results in failure through a particular degradation mode. These modes can be classified as thermal, mechanical, and chemical modes (3-7). Because fuel cells typically operate at temperatures below that at which Nafion thermally degrades, chemical and mechanical degradation are generally considered the most important modes of membrane degradation.Although there has been a lot of progress in the area, the exact chemical membrane degradation mechanisms, the most susceptible points, and the impact of test conditions on degradation are not completely understood. The formation of radicals and their reactivity during the fuel cell operation have been confirmed as the main cause of PEM chemical degradation (8-11). These radicals are formed as a result of the reaction of crossover H2 and O2 on the surface of Pt or other metal ion contaminants (4). In addition to the electrodes, Pt particles have been observed to migrate into the membrane, producing another site for radical formation. Radical attack leads to loss of fluorine, sulfur, and organofluorine compounds from the PEM, and PEM thinning, all resulting in loss of mechanical integrity. The radical formation mechanisms and the impact of test conditions on radical formation are also not completely understood.It has been suggested that earlier versions of Nafion membranes chemically degrade through radical attack on carboxylic acid groups end groups (12-14) or at the C−S or O−C bonds in the side chain and main chain (12, 14-18), as shown in Figure 1. Chemically stabilized Nafion, e.g., Nafion 211, has been developed where the concentration of terminal carboxylic acid groups was decreased to negligible levels and, as a result, degradation decreased substantially (13). The use of radical scavengers such as ceria have increased the durability of PFSA membranes by as much as 1000-fold compared to unmitigated membranes (19). Despite the recent durability improvements, there is evidence that PFSA membranes are still susceptible to chemical damage.Typical indicators for chemical degradation of Nafion membranes include increased H2crossover, decreased ion exchange capacity, decreased proton conductivity, and decreased PEM thickness. Localized crossover tests have shown that membrane damage occurs preferably at the perimeter of the active area. The quantitative method of choice used for detecting chemical degradation is typically detection of fluoride in fuel cell outlets, reported as a fluoride emission rate (FER).Figure 1. Proposed locations of radical attack in NafionTo improve PEM chemical durability, a true understanding of the decay mechanisms is required. Models of some of the decay modes have been developed and they show a consistent but incomplete understanding of membrane decay. Through a combination of specific modeling and directed testing, an overall decay model can be developed.

Solid Oxide Fuel Cells (SOFCs) are electrochemical devices with solid electrolyte materials that convert chemical energy in fuels into electrical energy. Tubular SOFC is one of the major SOFC configurations which has the advantages of good thermal conductivity, easy to seal and shorter start-up time. However, challenges remain in effectively collecting the current inside the tubular cells. Developing an efficient, easily applicable, and cost-effective collector is imperative. The properties of a desired current collector are to: reflect the fuel cell performance, be able to withstand the fuel cell’s high working temperature environment, protect the fuel cell from damage, simplify the current collector application procedure, expand the contact surface between the tube and the current collector, and save cost. We have designed a current collector and identified the materials for inside tube current collection with desired characteristics such as high temperature tolerance, low electrical resistance, and low cost. The research focused on the effectiveness of the current collector and its durability at various fuel cell operation conditions. The design achieved a peak power density of 0.21 W/cm2 at 0.7V and 700oC for a conventional tubular SOFC with material set of Ni+8YSZ‖YSZ‖LSM. Comparing to the general metal (nickel, silver) current collectors, the material which is provided in the paper can reduce the cost from hundreds of dollars per yard to $0.13/yard. The paper will discuss the design and the cost-effective material selection of the current collector which provides a potential path for the commercialization of tubular SOFC technology.

Solid Oxide Fuel Cells (SOFCs) are electrochemical devices with solid electrolyte materials that convert chemical energy in fuels into electrical energy. Tubular SOFCs (T-SOFCs) is one of the major SOFC configurations which has the advantages of good thermal conductivity, easy to seal and shorter start-up time. However, challenges remain in effectively collecting the current inside the tubular cells. Developing an efficient, easily applicable and cost-effective collector is imperative. The properties of a desired current collector are to: reflect the fuel cell performance, be able to withstand the fuel cell’s high working temperature environment, protect the fuel cell from damage, simplify the current collector application procedure, expand the contact surface between the tube and the current collector, and save cost.We have designed a current collector and identified the materials for inside tube current collection with desired characteristics such as high temperature tolerance, low electrical resistance, and low cost. The research focused on the effectiveness of the current collector and its durability at various fuel cell operation conditions. The design achieved a peak power density at 0.7V of 0.21W/cm2 at the temperature of 700oC for a conventional tubular SOFC with material set of Ni+8YSZ‖YSZ‖LSM. Comparing to the general metal (nickel, silver) current collectors, the studied material can reduce the cost from hundreds of dollars per yard to $0.13/yard. The paper will discuss the design and the cost-effective material selection of the current collector which provides a potential path for the commercialization of tubular SOFC technology.

In this work, we apply a nonlinear chaos analysis to the two phase flow in polymer electrolyte fuel cell cathode air-delivery microchannels. The fuel cell voltage signal is analyzed using techniques designed to estimate invariants typical of deterministic systems with high sensitivity to initial conditions, such as chaotic two phase flow. Voltage data are taken under varying fuel cell operating conditions, and noise in the data is reduced using a nonlinear noise reduction algorithm. The chaotic strange attractor of the system is reconstructed in phase space using time-delay embedding. Correlation sums over the strange attractor are calculated to estimate the fractal correlation dimension of the system. Estimations of the Kolmogorov entropy provide an additional measure of the complexity of the strange attractor. The values of the chaotic invariants are compared across varying degrees of cathode flooding to discern how they change with two phase flow regimes and fuel cell operating conditions. Future work will involve leveraging the chaotic understanding of two phase flow with chaos control methods to increase the power stability.

This paper summarizes some of the progress that has been made over the past 30 years in identifying chemical and mechanical degradation mechanisms in proton-exchange membrane fuel cells (PEMFCs), as well as approaches in refining stack operating conditions, cell hardware configurations and membrane- electrode assembly (MEA) stabilization to mitigate degradation and extend life. New analytical methods have been brought to bear to understand the relationships between PEM degradation and fuel cell operating conditions, especially low relative humidity, elevated temperatures, and frequent open-circuit periods. PEM/MEA degradation similar to that observed in PEMFC may also occur under certain accelerating conditions for proton-exchange membrane water electrolyzers (PEMELCs) that typically operate for 30 to 100 thousand hours without incident when properly hydrated, thermally managed and mechanically supported. A comparison is made of common hardware and operational factors for PEMELCs and PEMFCs that can lead to PEM/MEA degradation.

A mathematical model for predicting the life of polymer electrolyte fuel cell membranes subjected to hydration cycling

The effect of inverter ripple (feedback on a fuel cell from the DC to AC inverter) current on solid oxide fuel cell performance is not well understood. At NETL, both models and experimental work are being pursued to try and understand the effect of inverter ripple on cell performance. The modeling work has led us to speculate that large oscillations in voltage due to the inverter may have deleterious effects on cell performance and lifetimes. Results showed the behavior of the reactant concentrations within the stack electrodes under inverter loads with frequencies between 60 Hz and 1250 Hz. The conclusion was that due to the fluctuating reactant concentrations, temporary conditions (on the order of the frequency of ripple) would occur where the concentration of H 2 at the anode/electrolyte interface varied significantly from the average value. Since much work in the development of fuel cells relies on steady state test data, concern is raised that final operation on an inverter would have significantly different performance results. Here we report the results of experimental tests of the effect of ripple on solid oxide fuel cells. Data was acquired over several weeks on the performance of an individual cell while being exposed to 30% ripple current at a frequency of 60Hz. No degradation that could be attributed to inverter ripple was detected over the period of testing.

This study quantitatively describes the carbonate- and bicarbonate-forming reaction mechanism in an operating alkaline exchange membrane fuel cell that occurs as a result of carbon dioxide in the cathode gas stream. A spatially averaged transient theoretical model was created for this study and validated to experimental data found in the literature. Results present the prediction of the membrane’s ionic conductivity as a function of operating conditions and membrane properties. The self-purging phenomenon was observed and studied, as well as the emission of carbon dioxide from the membrane during operation. Following the conductivity study, suggestions can be made for optimal operating conditions and membrane properties to improve fuel cell performance in the presence of carbon dioxide.

Nafion has been dominating the proton exchange membrane (PEM) market because of its adequacy for the proton exchange operation. However, because of its high cost, low performance at low relative humidity, and finally environmental unfriendliness, researchers have been developing and working on Nafion alternatives. In this review, we assess how biopolymers present an opportunity as an eco‐friendly material able to replace the conventional Nafion membrane in proton exchange membrane fuel cells (PEMFC). Chitosan and cellulose were the most studied in literature owing to their abundance in nature, low cost, and high affinity for water, which are sought‐after properties for PEM. In addition to that, their ease of modification through their hydroxyl or amine groups represents an opportunity to further enhance their characteristics and thus meet the criteria of diverse applications. The use of biopolymers as an adequate PEM is facing many challenges. Having a practical proton conductivity and securing the physicochemical stability of the biopolymers in fuel cell operating conditions are two of the most important challenges to overcome. Promising strategies to simultaneously address these challenges such as crosslinking, making interpenetrated networks, and making blends and composites are reported.

Blend membranes based on sulfonated polyetheretherketone (SPEEK) and containing 5 wt.% of silylated and sulfonated polyphenylsulfone (SiSPPSU) were prepared and characterized by attenuated total reflectance Fourier transform infrared spectroscopy (ATR/FTIR) and by electrochemical impedance spectroscopy (EIS). Tests in a direct methanol fuel cell station were also performed. The experimental results demonstrated that the presence of hybrid polymer SiSPPSU allowed to increase the proton conductivity of electrolyte and to improve its performance under fuel cell operating conditions. Due to its satisfactory properties, SPEEK/ SiSPPSU blend membrane has a potential use in DMFCs operating at intermediate temperatures.

The electrical resistance of the collector plate (both bulk and surface resistance) plays a major role in fuel cell stack performance. Energy Partners (EP) has developed molded graphite composite plates with enhanced electrical conductivity, as an alternative to expensive machined graphite. By optimizing collector plate materials, compounding and molding processes, the resistance of the molded collector/backing layer sandwich was reduced by about 12 mΩcm2. This resulted in an improvement in performance of more than 50 mV per cell at 1 A/cm2 in a 10-cell stack with 300 cm2 active area.

A two-dimensional model of a proton exchange membrane fuel cell stack is developed. Taking advantage of the geometrical periodicity of the stack, the model is used to predict the detailed thermal and electrochemical characteristics of the fuel cell. Using recently reported as well as new experimental results, the electrical and thermal contact resistances and modifications in the gas diffusion layer transport properties that develop within the stack in response to changes in the compressive force used to assemble the stack are accounted for. The fuel cell stack performance, reported in terms of its power output and internal temperature distributions, is very sensitive to the compressive load.

Investigation of water management in polymer electrolyte membrane fuel cells (PEMFCs) is a fundamental and important issue for their implementation. Consequently, it is necessary to investigate water content distribution in PEMs under typical fuel cell operating conditions. Our research group has therefore investigated transversal water content distribution in PEMs under fuel cell operating conditions using a magnetic resonance imaging (MRI) system. However, the spatial resolution limitations impeded our prior studies to elucidate the distribution of water contents in a thin membrane. We investigated water content distribution in a thin membrane using a high spatial resolution magnetic resonance imaging (MRI) system to overcome spatial resolution limitations in a thin membrane in this study. Transversal water content distribution in PEM (e.g. Nafion 115) was investigated at room temperature and ambient pressure using a high spatial resolution (e.g. < 10 micrometer) MR imaging system.