Performance Of Polymer Electrolyte Membrane Research Articles

Application of Carbon Felt as a Flow Distributor for Polymer Electrolyte Membrane Fuel Cells

In this work, carbon felt is employed as a flow distributor in polymer electrolyte membrane fuel cells and the fuel cell performance of this design is compared to that of a conventional graphite serpentine channel design and copper metal foam. Experimental results show that carbon felts as flow distributors offer advantages over other flow distributors in terms of enhanced rates of mass transfer due to their levels of high porosity and strong convective flow. As a result, fuel cells with carbon felt as flow distributors perform well and especially in high current regions. In addition, the effects of the hydrophilicity of carbon felt on fuel cell performance are evaluated. The hydrophobic surface of raw carbon felt is rendered hydrophilic via ozone treatment. Ozone-treated hydrophilic carbon felt helps remove water from the hydrophobic gas diffusion layer, decreasing mass transfer resistance. However, the formation of oxygen functional groups by ozone treatment increases the levels of ohmic resistance. Thus, it is necessary to develop highly conductive and hydrophilic carbon felt as a flow distributor for high performance of polymer electrolyte membrane fuel cell.

Visualization of liquid water in a lung-inspired flow-field based polymer electrolyte membrane fuel cell via neutron radiography

Lung-inspired, fractal flow-fields hold great potential in improving the performance of polymer electrolyte membrane fuel cells (PEMFCs) by providing uniform gas distribution across the electrodes and ensuring minimum entropy production in the whole system. However, the inherent susceptibility of the fractal flow-fields to flooding renders their use inadequate at high humidity conditions. In-depth understanding of water management in lung-inspired flow-fields is indispensable for the implementation of alternative outlet channel geometries or engineered water removal strategies to alleviate flooding. Here, liquid water formation and transport across the lung-inspired and serpentine flow-field based PEMFCs are evaluated using neutron radiography. The results reveal a propensity to flooding in the interdigitated outlet channels of the fractal flow-field with N = 4 generations as a result of slow gas velocity and narrow channel dimensions, which leads to significant performance deterioration. Neutron images also elucidate the importance of ensuring a well-defined internal channel structure of the fractal flow-fields to prevent backflow of liquid water via wicking and capillary pressure build-up arising from the narrow inlet gas channels and hydrophobic gas diffusion layer.

Enhancing under-rib mass transport in proton exchange membrane fuel cells using new serpentine flow field designs

New flow field configurations are developed to improve the performance of polymer electrolyte membrane fuel cells (PEMFCs). The developed designs aim to uniformly distribute the reactants over the reaction area of the catalyst layer surface, boost the under-rib convection mass transport through the gas diffusion layer, decrease the water flooding effect in the gas diffusion layer-catalyst layer interface, and maintain the membrane water content within the required range to augment protonic conductivity. To evaluate the performance parameters of a PEMFC, a comprehensive three-dimensional, two-phase mathematical model has been developed. The model includes the charge transport, electrochemical reactions, mass conservation, momentum, energy, and water transport equations. The results signify that the improved flow field patterns attain a considerable boosting of the output power, the under-rib convection mass transport, improvement of the reactant distribution over the catalyst layer surface and decline of the liquid water saturation in the gas diffusion layer-catalyst layer interface. The developed configurations achieve a higher power density of 0.82 W/cm2 at a current density of 1.74 A/cm2, compared to the standard serpentine configuration, which attains about 0.67 W/cm2 at a current density of 1.486 A/cm2.Accordingly, the develop configurations demonstrate a 22.6% enhancement in power density.

Examining the effect of the secondary flow-field on polymer electrolyte fuel cells using X-ray computed radiography and computational modelling

Flow-fields are key factors in determining the operation of fuel cells. While extensive work has been conducted to develop and optimise the reactant flow and current collection performance of polymer electrolyte membrane fuel cell (PEMFC) components, there is a factor that remains largely unaccounted for. Depending on how a membrane electrode assembly (MEA) is incorporated into a cell, there will often be a small gap between the edge of the gas diffusion layer (GDL) and the seal or bipolar plate. This gap acts as a ‘secondary flow-field’ (SFF) that can bypass or affect/augment the conventional or ‘primary flow-field’. Understanding how this affects performance (either positively or adversely) is essential for holistic flow-field design. This paper describes the issues associated with the SFF, examines how cell compression affects its width due to lateral expansion of the GDL and discusses the results of a 3-D computational model that investigates the effect of the SFF during dead-ended anode (DEA) operation for a fuel cell without a macroscopic (conventional) anode flow-field.

Correlation between performance of polymer electrolyte membrane fuel cell and degradation of the carbon support in the membrane electrode assembly using image processing method

Long-time operation and various conditions cause the membrane electrode assembly (MEA) of polymer electrolyte membrane fuel cells (PEMFCs) to degrade, which results in decreased performance. The degradation of the MEA appears as various symptoms, such as the loss of carbon support and agglomeration of the Pt catalyst. In this paper, damage on the surface of the MEA by long-time operation and various conditions is induced intentionally by high-temperature conditions in a thermostat chamber. The MEA surface damage is photographed by scanning electron microscopy (SEM), and the loss of the carbon support that fixes the platinum catalyst is judged. Image processing is used to analyze damage on the MEA surface, and binarization processing is applied to the image processing method. SEM imagery is taken at magnifications of 100 × and the trends in quantified surface damage on the MEA according to the degradation temperature are analyzed. The correlation between the quantitative damage on the MEA surface and the performance of the PEMFC is checked. As a result, the tendency of decreasing PEMFC performance is derived from increasing quantified damage on the MEA surface.

(Invited) Fuel Cell Contamination

As fuel cell systems become more commercially competitive, and as automotive fuel cell research and development trends toward decreased catalyst loadings and thinner membranes, fuel cell operation becomes even more susceptible to contaminants. Contaminants derived from fuel cell system components such as structural materials, lubricants, greases, adhesives, sealants, and hoses have been shown to affect the performance and durability of fuel cell systems. Between July 2009 and September 2013, the National Renewable Energy Laboratory (NREL) led a team to study the effect of system contaminants on the performance of polymer electrolyte membrane fuel cells. The team, made up of industry (GM), universities (University of South Carolina, University of Hawaii, and Colorado School of Mines (CSM)), and a national laboratory partner (Los Alamos National Laboratory), screened about 60 balance of plant (BOP) materials. Knowledge of the material contamination potential of various system components can be used by the fuel cell industry in selecting appropriate BOP materials and in cost-benefit analyses. Our goal was to increase the understanding of fuel cell system contaminants and to help guide the implementation and, where necessary, development of system materials that will help enable fuel cell commercialization. NREL and CSM also studied the effect of membrane degradation model compounds of commercial Nafion and 3M commercial perfluorosulfonic acid (PFSA) membranes. Such compounds are derived from PFSA membranes when exposed to certain peroxides and hydroxyl radicals.1,2 Two compounds in particular, perfluoro(2-methyl-3-oxa-5-sulfonic pentanoic) acid (DA-Naf) and perfluoro(4-sulfonic butanoic) acid (DA-3M), arise along with HF as the main membrane degradation compounds of Nafion and a 3M commercial PFSA membrane, respectively.3.4 These degradation products have been shown to adsorb on Pt-based electrocatalysts, leading to a loss in catalyst electrochemical surface area (ECA), oxygen reduction reaction (ORR) activity, or both.5,6 Other model compounds were also studied to gain fundamental insight on the adsorption effects solely due to sulfonic acid functional group, the carboxylic acid functional group, as well as that of the fluorocarbon chain length. Recently, NREL and 3M examined the effect of perfluoro imide acid (PFIA) ionomer degradation products on the ORR activity by performing a series of rotating disc electrode (RDE) experiments. PFIA is a new multi-acid side chain ionomer that 3M Fuel Cell Components Group is developing for fuel cell applications, with the goal of improved conductivity compared to Nafion when operated at dry conditions. During an accelerated stress test (open circuit voltage (OCV) hold), 3M reported that a PFIA-containing fuel cell showed a decay in the OCV in the first 200 h and then an increase in the membrane resistance for the remaining 600 h.7 It is suspected that the oxidative degradation of the PFIA polymer may be similar to that of the PFSA systems. Three model compounds were provided by 3M and systematically studied at NREL with respect to the effect of the potential PFIA degradation products on ORR activity. This presentation will briefly summarize the past efforts related to contaminants derived from fuel cell system components, along with the interactive material screening data tool that was developed to archive the results from these studies and make them publicly available,8 as well as the RDE studies of PFSA and PFIA membrane degradation model compounds. Acknowledgement: This work was supported by the U.S. Department of Energy (USDOE), Office of Energy Efficiency and Renewable Energy (EERE), Fuel Cell Technologies Office (FCTO) under Contract No. DE-AC36-08GO28308 with Alliance for Sustainable Energy, LLC, the Manager and Operator of the National Renewable Energy Laboratory. Model compounds were provided by 3M for this study.

Ionic Conductivity and Cycling Stability Improvement of PVDF/Nano-Clay Using PVP as Polymer Electrolyte Membranes for LiFePO₄ Batteries.

In this paper, we present the characteristics and performance of polymer electrolyte membranes (PEMs) based on poly(vinylidene fluoride) (PVDF). The membranes were prepared via a phase-inversion method (non-solvent-induced phase separation (NIPS)). As separators for lithium battery systems, additive modified montmorillonite (MMT) nano-clay served as a filler and poly(vinylpyrrolidone) (PVP) was used as a pore-forming agent. The membranes modified with an additive (8 wt % nano-clay and 7 wt % PVP) showed an increased porosity (87%) and an uptake of a large amount of electrolyte (801.69%), which generated a high level of ionic conductivity (5.61 mS cm−1) at room temperature. A graphite/PEMs/LiFePO4 coin cell CR2032 showed excellent stability in cycling performance (average discharge capacity 127 mA h g−1). Based on these results, PEMs are promising materials to be used in Polymer Electrolyte Membranes in lithium-ion batteries.

The Change of Water Distribution in Porous Media of the Polymer Electrolyte Membrane Fuel Cell after Freeze/thaw Cycles

AbstractEffect of freeze/thaw cycles on the performance of polymer electrolyte membrane fuel cell (PEMFC) is investigated. Freeze/thaw cycle is repeated 40 times. The performance is degraded, as the number of freeze/thaw cycles increases. The maximum degradation is about 35% at high current density. The performance degradation is analyzed based on the change of water distribution. X‐ray visualization is conducted 3 times; before, after 10th and 40th freeze/thaw cycle, respectivly. The water saturation in PEMFC is getting reduced, as the number of freeze/thaw cycles increases. Over‐potential analysis is conducted using visualization result by calculating the proton conductivity. The proton conductivity is decreased about 30% at high current density after 40th freeze/thaw cycle. The change of water distribution is analyzed based on the structural change of the GDL. The water volume change upon freeze/thaw cycle leads to cracks in the micro porous layer. It facilitates water removal from the GDL and leads to low water saturation in PEMFC.

System identification black box approach for modeling performance of PEM fuel cell

Abstract A polymer electrolyte membrane (PEM) fuel cell is very useful for distributed generation and for portable users like electric vehicles because it is very efficient, emission free and operated at low temperature. However, it is not so easy to find direct experimental estimates of the actual performance of PEM fuel cell through various phenomena and operating conditions like chemical reactions, fuel pressure, working temperature and fuel humidity. Mathematical modeling, therefore, plays an important role in understanding operational performance of PEM fuel cell. The actual operating performance of PEM fuel cell depends on a number of parameters, therefore developing an accurate model that includes its dynamic behavior is most important. In this paper, first time, the system identification black box approach is used to develop a number of simple but more realistic mathematical model structures for a PEM fuel cell. The performance of each model structure is compared with the data from a 25 cm2 active area practical PEM fuel cell for result validation. The presented models can be used to predict polarization behavior of the PEM fuel cell under different loading conditions.

The effect of cathode nitrogen purging on cell performance and in operando neutron imaging of a polymer electrolyte membrane electrolyzer

Through-plane in operando neutron radiography was used to differentiate between anode feed water and cathode water (transported through the membrane) to resolve oxygen gas bubbles in the anode. In this work, we investigated the impact of cathode purging on the electrochemical performance of polymer electrolyte membrane (PEM) electrolyzers as well as its impact on neutron radiographic imaging to inform our recommendations on how to prescribe an optimal cathode purging rate. We found that excessive dry nitrogen purging in the cathode led to an increase in ohmic resistance and a decrease in the open circuit voltage. Therefore, an optimal nitrogen purging rate must be prescribed during visualization studies to maximize the precision of oxygen gas quantification without unintentionally altering the electrochemical performance. We demonstrated that the optimal cathode nitrogen purge rate can be determined by monitoring the spatial distribution of the anode oxygen gas and that this optimal purge rate is a function of the operating current density.

Tuning the Ionomer Distribution in the Fuel Cell Catalyst Layer with Scaling the Ionomer Aggregate Size in Dispersion.

With the demands for better performance of polymer electrolyte membrane fuel cells, studies on controlling the distribution of ionomers have recently gained interest. Here, we present a tunable ionomer distribution in the catalyst layer (CL) with dipropylene glycol (DPG) and water mixtures as the ionomer dispersion medium. Dynamic light scattering and molecular dynamics simulation demonstrate that, by increasing the DPG content in the dispersion, the size of the ionomer aggregates in the dispersion is exponentially reduced because of the higher affinity of DPG for Nafion ionomers. The ionomer distribution of the resulting CLs dictates the dimensional feature of the ionomer dispersion. Although the ionomer distribution becomes more uniform with increasing the DPG content, an optimal power performance is obtained at a DPG content of 50 wt % regardless of feed humidity because of balanced proton and mass transports. As a guide for tuning the ionomer distribution, we suggest that the ionomer aggregates in the dispersion with a size close to that of the Pt/C aggregates form a highly connected ionomer network and maintain a porosity in the catalyst/ionomer aggregate, resulting in high power performance.

Layer‐by‐layer Proton Donor and Acceptor Membrane for an Efficient Proton Transfer System in a Polymer Electrolyte Membrane Fuel Cell

AbstractThe high performance proton transfer in polymer electrolyte membranes (PEMs) is known to be achieved through cooperating of proton donors and acceptors. In order to obtain the effective and efficient proton transfer, the present work proposes a membrane with proton donors and proton acceptors in layer‐by‐layer (LbL) structure. By applying sulfonated poly (ether ether ketone) (SPEEK) as the substrate membrane and soaking alternatively with the proton donor polymer, i.e. poly(acrylic acid) (PAA), and the proton acceptor heterocycles, i.e. benzimidazoles (BI), the LbL SPEEK/BI/PAA membranes can be achieved. The systematic variation based on the branching benzimidazole groups suggest that the LbL SPEEK/BI/PAA membrane obtained shows an increase in proton conductivity compared to the neat SPEEK, especially in the case of the membrane with trifunctional benzimidazole (B4) is as high as 1–2.5 orders of magnitude over the temperature range of 30–170 °C. The present work points out the simple surface modification of the proton conductive membrane by layer‐by‐layer approach as a practical and simple way to enhance the performance of polymer electrolyte membrane.

Effect of Pt loading and catalyst type on the pore structure of porous electrodes in polymer electrolyte membrane (PEM) fuel cells

Catalyst layer (CL) has a significant impact on the overall pore structure of the entire electrodes, thereby impacting the transport processes and the performance of polymer electrolyte membrane (PEM) fuel cells. In this study, the contribution of the CL to the entire electrode structure is experimentally investigated. The electrodes are prepared by using two types of catalysts with different platinum/carbon (Pt/C) ratios and Pt loadings and characterized by the method of standard porosimetry (MSP). The results show that for the same type of catalysts, as the Pt loading is increased, both the porosity and mean pore size of the electrode decrease, whereas the pore surface area increases. For a constant Pt loading, a lower Pt/C ratio results in a thicker electrode with a smaller porosity, smaller pore size, and larger pore surface area. The fractal dimension is found to be a good representative of the complexity of the pore structure of the electrode; a larger fractal dimension is detected for a higher Pt loading and a smaller Pt/C ratio.

Effect of Chain Architectures of Star-shaped Poly(ethylene glycol) Macromonomers on Enhancement of Thermal, Mechanical, and Electrochemical Performance of Polymer Electrolyte Membranes

Polymer electrolyte membranes (PEM) have been fabricated from various star-shaped poly(ethylene glycol) macromonomers to investigate the effect of chain-branching architectures on thermal, mechanical, and electrochemical properties. The PEM prepared from the 4-arm star macromonomer network revealed the lowest glass transition temperature (Tg), the highest modulus, elongation-to-break, and tensile strength relative to those prepared from 3-arm star and linear macromonomers. On the same token, the PEM based on the 4-arm PEG showed the highest ionic conductivity and good capacity retention.

Tailoring the Membrane‐Electrode Interface in PEM Fuel Cells: A Review and Perspective on Novel Engineering Approaches

AbstractThe interface between the catalyst layer (CL) and the polymer electrolyte membrane (PEM) in a fuel cell has substantial impact on its electrochemical performance. In consequence, there have been growing research activities to engineer this interface to improve the performance of polymer electrolyte membrane fuel cells (PEMFCs). This review summarizes these novel approaches and compares the various techniques. Based on available fuel cell data in the literature, a quantitative comparison of relative improvements due to a micro‐ and nano‐engineered PEM|CL interface is provided. This allows several conclusions: First, regardless of the applied method, a re‐engineering of the PEM|CL interface leads to an improvement of power‐determining parameters, such as mass transport resistances. The latter has hitherto not been clearly connected to the PEM|CL interface and is an important piece of information for future fuel cell development. Second, for patterned membrane surfaces, feature sizes of about 1–10 µm on the membrane surface seem to result in the most significant power density improvement. Third, an engineered PEMCL interface can contribute to extend the fuel cell durability due to enhanced adhesion and contact between the two layers. With this, novel membrane electrode assemblies (MEAs) can be designed that enable significantly higher power densities compared conventional 2D‐layer MEAs.

Effect of height/width-tapered flow fields on the cell performance of polymer electrolyte membrane fuel cells

Water flooding at the cathode is a critical issue in polymer electrolyte membrane fuel cells, since it will block the oxygen transport, resulting in a large concentration loss. To address this issue, two tapered flow fields with varying height or width are proposed in this study, as the tapered channels can gradually increase flow velocity which is beneficial in water removal. To confirm the proposed function, both experiments and simulation are performed. The results prove that the tapered flow fields with high velocity at the downstream region can significantly enhance water removal in flow channels, which avoids the mass transport limitation and improves the cell performance at high current densities. Additionally, the tapered design can also enhance the under-rib convection between adjacent channels, which helps to remove accumulated water in the gas diffusion layer.

The effect of compression molding parameters on the electrical and physical properties of polymer composite bipolar plates

The performance of polymer electrolyte membrane fuel cell (PEMFC) greatly depends on the properties of the components. Bipolar plate is one of the key components of PEMFC and the properties of the plates on fuel cell performance are critical. Electrical conductivity and surface roughness of the bipolar plate appear very important properties to minimize ohmic resistance and optimize water management, respectively. The composite bipolar plates having conductive fillers and thermosetting resin binder are produced by compression molding. Response Surface Methodology (RSM) has been applied to optimize the production conditions of bipolar plate. Electrical conductivity, physical appearance and roughness are chosen as response parameters and molding temperature, pressure and time are chosen as independent parameters. The main challenges of this study are observation of the individual and combined effects of these parameters on bipolar plate properties by RSM. The optimization of the production conditions of polymer composite plate is carried out by aiming maximum electrical conductivity and minimum time. The maximum electrical conductivity of 107.4 Scm−1 is obtained at temperature of 187 °C, pressure of 119 bar and time of 5 min. It is found that the polymer composite plates produced by the compression molding process at minimum time satisfied the electrical conductivity target of Department of Energy (DoE).

Gradient design of Pt/C ratio and Nafion content in cathode catalyst layer of PEMFCs

In order to increase the utilization of Pt, reduce mass transfer loss and improve the performance of polymer electrolyte membrane fuel cells (PEMFCs) under low humidity and high current densities, the cathode catalyst layers with two layers of different Pt/C ratio and Nafion content are fabricated and evaluated. Polarization curves (IVs), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) are employed to characterize and compare the effects of Pt/C ratio and Nafion content gradient on the performance of PEMFCs under different humidification conditions. The results indicate that the performance of the membrane electrode assembly (MEA) can be significantly improved via allocating more Nafion and Pt/C in the sublayer near the membrane in cathode catalyst layer. The MEA with optimal gradient cathode catalyst layer results in improved catalysts utilization compared to MEA with single cathode catalyst layer, 0.403 g kWrated−1 and 0.711 g kWrated−1 under 80 RH% and 20 RH%, respectively. The areal power density of the optimal MEA is 28.4% and 135.7% higher than the conventional single-layer catalyst layer MEA under high and low humidity, respectively.

Development of contact pressure distribution of PEM fuel cell's MEA using novel clamping mechanism

Clamping mechanisms have significant effect on the performance of polymer electrolyte membrane (PEM) fuel cells. In this paper, PEM fuel cell with new clamping mechanism is designed to study the contact pressure distribution over the active area of PEM fuel cell's membrane electrode assembly (MEA). The clamping pressure is pneumatically exerted on the PEM fuel cell assembly. A comparison between the conventional and new clamping mechanism is carried out with simulation, and the numerical results are validated against experimental investigation performed in the fuel cell technology research laboratory. The experimental results are gathered using embedded pressure measurement films in the designed single cell. The results achieved via finite element method are in good agreement with experimental results. It is concluded that the contact pressure distribution of MEA for the new clamping mechanism is more uniform than the conventional one.

PEMFC modeling based on characterization of effective diffusivity in simulated cathode catalyst layer

Oxygen diffusion in the cathode catalyst layer (CCL) is crucial to the high performance of polymer electrolyte membrane fuel cells (PEMFCs), especially in high current density or concentration loss regions. Recently, PEMFC performance has been reported to be enhanced by increasing CCL pore size and pore volume due to the reduction of diffusion resistance by capillary water equilibrium [Yim et al., Electrochimica Acta 56 (2011) 9064–9073]. Herein, we simulate these experimental results utilizing a new one-dimensional PEMFC model considering the effects of accumulated water film in CCL on oxygen diffusion. Two CCL microstructures were numerically generated based on agglomerate models to examine the experimental results obtained for two membrane electrode assembly (MEA) samples with different CCL porosity. The effective diffusivity of oxygen in the CCL was estimated by performing auxiliary simulations of oxygen concentration in CCL microstructures covered by a film of liquid water, with exponential correlation obtained between effective diffusivity and the thickness of the above film. Polarization curves predicted by the present model were in good agreement with experimental results. In agreement with the results of Yim et al., the present model predicts that the MEA featuring a CCL with smaller pores (which are more easily filled by liquid water) should exhibit a larger concentration loss at high current densities.