PEMFC Performance Research Articles

Influence of Short-Side-Chain Perfluorosulfonic Acid Ionomer As Binders on the Performance of Fuel Cell Cathode Catalyst Layers

The performance of PEMFCs has been linked to the porosity of the catalyst/carbon/ionomer composite in the cathode catalyst layer (CL), and this can be controlled to some degree by the equivalent weight (EW) of the ionomer.(1) Here, we study impact on PEMFC performance when the cathode CL contains a low equivalent weight (LEW) ionomer having short-side-chains (SSCs). Inks for the CLs are prepared using Pt/C electrocatalysts and either 27 wt.% Aquivion® SSC perfluorosulfonic acid ionomers (i.e. Aquivion® ionomer 980 EW, 830 EW and 700 EW) or a standard 33 wt.% long-side chain (LSC) Nafion® 1100 EW ionomer. The Pt/C/ionomer ink is directly deposited onto the Nafion proton exchange membrane by direct ultrasonic spray deposition to form catalyst-coated membranes (CCMs). The effects of ionomer EW on the surface area, pore structure and morphology of the prepared cathode CLs are surveyed by nitrogen adsorption and scanning electron microscopy (SEM). The CCMs are assembled into membrane electrode assemblies (MEAs), and the performance of the resulting MEAs are analyzed by I-V polarizations at different cell operating temperatures (i.e. 95, 80, 70 and 40°C) and relative humidity (i.e. 100, 70, 50 and 25% RH) in both air and oxygen environments. Cyclic voltammetry is used to determine the cathode CLs Pt ECSA and Pt utilization. In situ electrochemical impedance spectroscopy is used to understand the polarization losses in the cells, particularly in the mass transport region where electrode porosity has a large impact on flooding. Figure 1 compares the average polarization curves measured for 5 MEAs employing the SSC ionomer in their cathode CLs (i.e. Aquivion® 830 EW) to 5 MEAs employing the LSC ionomer in their cathode CLs (i.e. Nafion® 1100EW) at 80°C fed with ambient pressure air humidified at 100 % and 50 % RH. In the kinetic region (i.e. E ≥ 0.80 V) of the polarization curves, the cell performance of both set of MEAs are identical at both 100 and 50% RH. In the mass transport region at high current densities, the MEAs containing the SSC ionomer in their cathode CLs perform better than those containing the LSC ionomer in their cathode CLs under both moderate (50%) and high (100%) relative humidity. Our results appear to be unique because others have only seen improvement in PEMFCs with SSC LEW ionomers under low RH conditions, while we see performance improvements from the LEW SSC ionomers across the full range of RH. (1, 2) This presentation will discuss how the SSC ionomer affects the physical attributes of the MEA (porosity, impedance), resulting in higher performance at high current densities independent of the RH. Figure 1. Comparison of the average polarization curves measured for 5 Pt/VC MEAs employing the SSC ionomer (Aquivion® 830EW) in their cathode CLs to 5 Pt/VC MEAs employing the LSC ionomer (Nafion® 1100EW) in their cathode CLs at 80°C fed with ambient pressure air humidified at 50% and 100% RH. 1. C. Lei, D. Bessarabov, S. Y. Ye, Z. Xie, S. Holdcroft and T. Navessin, J Power Sources, 196, 6168 (2011). 2. Y. C. Park, K. Kakinuma, H. Uchida, M. Watanabe and M. Uchida, J Power Sources, 275, 384 (2015). Figure 1

A numerical study on thermal analysis and cooling flow fields effect on PEMFC performance

Thermal management and water management are two important interconnected topics in the design and increase the efficiency of PEM fuel cells. Suitable cooling flow field design with proper performance is an important factor in increasing the lifetime of PEM fuel cell, because non-uniformity of temperature reduces the stability and durability of PEM fuel cell. Different cooling strategies are considered for removing of heat generation by PEM fuel cell, because the fuel cell temperature remains in a tolerable range and homogeneous as possible. In the first step, determine the value and location of heat sources in fuel cell, is important and appropriate cooling strategy can be applied. In this study, a PEM fuel cell with serpentine gas flow field is simulated with six different cooling flow fields simultaneously, e.g. conventional serpentine (Model 1), typical of MPSFFs (model 2), typical of serpentine (Model 3), parallel-serpentine (Model 4), conventional spiral (Model 5) and conventional parallel (Model 6). This simulation showes a correct perception of temperature distribution in PEMFC. The results indicate that Model 5 has a good temperature and performance based on the minimum and maximum temperature gradient, Index of uniform temperature (IUT), however has a more pressure drop. For second choice when the pressure drop is important, the Model 3 has a better performance than other models. Also, thermal analysis in these cases shows that ohmic, entropic and reaction heating included 20%, 35%, 45% of the total heat generation by PEMFC, approximately.

Finite Thickness Effects on Nafion Water Uptake and Ionic Conductivity at Hydrophilic Substrate Interfaces, and Implications for PEMFC Performance

The effect of film thickness on water uptake in Nafion films is probed via neutron reflectometry for a series of 10 samples with thicknesses ranging from 7–103 nm, and the resulting effects on ionic conductivity. The water uptake in the lamellae and the bulk-like layer varies non-monotonically, decreasing as the equivalent Nafion thickness increases from 7 nm to 12 nm, increasing with increasing film thickness for equivalent Nafion thicknesses ranging from 12 nm—60 nm, and then constant for films with equivalent thickness ≥ 60 nm. Composition depth profiles are used to predict the anisotropic ionic conductivities, which are used in a flooded-agglomerate model of PEMFC catalyst layers to predict the effect of lamellar structure transport properties on PEMFC performance.

Study of Cathode Catalyst Layer Parameters for HT-PEMFC Using Electrochemical Impedance Spectroscopy

We investigate the effect of platinum loading, cathode catalyst layer thickness and the phosphoric acid doping level on the performance of a polybenzimidazole based HT-PEMFC MEA. The objective is to characterize the cathode catalyst layer, which is known to be the limiting factor for PEMFC performance due to the sluggish kinetics of the oxygen reduction reaction. The range for each parameter is chosen carefully to investigate cell performance. Impedance measurements at four current densities (10, 20, 50 and 100 mA cm-2) with high cathode stoichiometry are performed. These operating conditions minimize the mass transport losses and therefore, the impedance data can be fitted with a simple equivalent circuit model to extract electrochemical parameters related predominantly to the cathode catalyst layer. These parameters are presented for eight membrane electrode assemblies and the cell performances are discussed. Finally, a comparison between the parameters is made and directions for improvement are suggested.

Effect of MPL Properties on Temperature and Liquid Water Distribution in a GDL

Management of heat and liquid water transport is important for further improvement of PEMFC performance. In this study, the effect of MPL properties on temperature and liquid water distribution was investigated. An X-ray CT apparatus was used for visualizing the liquid water in the GDL, and Au - Ni thin film thermocouples with a thickness of about 9 μm were fabricated using MEMS technology and used for the temperature measurement.

Strategies to mitigate Pt dissolution in low Pt loading proton exchange membrane fuel cell: I. A gradient Pt particle size design

The catalyst durability at low Pt loading remains a barrier for industrial commercialization of the proton exchange membrane fuel cell. Degradation of low loaded Pt catalyst not only reduced the electrochemical surface area but also revealed a Pt depletion zone adjacent to the cathode/membrane interface where about 80% of the Pt was lost due to dissolution and migration into the membrane. We hypothesized that the Pt degradation can be mitigated using a gradient cathode design without sacrificing the initial fuel cell performance. With a focus on the mitigation, the first of two companion papers focuses on the Type I cathode with larger Pt particles (5nm average size) near the cathode/membrane interface. In the second of these two papers, the Type II cathode with higher Pt loading (60wt.% Pt on carbon) near the cathode membrane interface will be investigated. The catalyst coated membranes with gradient cathode were fabricated by reactive spray deposition technology with low Pt loading of 0.05mgcm−2 on the anode and 0.1mgcm−2 on the cathode. The DOE defined accelerated stress test was performed by imposing a triangular wave potential cycling from 0.6V to 1.0V for 30,000 cycles at 50mVs−1 scan rate. Results of accelerated stress test showed that the loss of electrochemical surface area and PEMFC performance were reduced for the Type I cathode compared to the control cathodes that consisted of 2nm averaged Pt particle size only. Microscopy analysis suggested two possible mechanisms for this durability improvement: 1) the reduction of the amount of Pt lost due to Pt dissolution, and 2) the preservation of Pt particles at the cathode/membrane interface. Comparison with the control cathode showed that the loss of Pt near the cathode/membrane interfaced were reduced from 80% to 40% and the overall Pt loss of the cathode were reduced from 30% to 15%.

Effect of number and arrangement of separator electrode assembly (SEA) on the performance of square tubular PEM fuel cells

The effect of intermediate electrode on the performance of the square tubular PEMFC was studied in the present paper. Four different configurations were developed and evaluated for the first time, namely: simple square tubular (without intermediate electrode i.e. Simple), double parallel intermediate electrode (DPIE), double bisectors intermediate electrode (DBIE) and triple parallel intermediate electrode (TPIE). Calculations were performed maintaining the same active area and boundary conditions for all four configurations. The results of polarization curves showed that the insertion of the intermediate electrode increases current density, and leads to higher consumption of reactants and better performance of PEMFC. Employing the intermediate electrode not only does not cause any additional cost compared to simple square tubular configuration, but also the cost is reduced in comparison to the base model due to the reduction of fuel cell length. The results also revealed that the intermediate electrode arrangement also has a significant influence on the performance of PEM fuel cell; For instance, DPIE compared to DBIE delivers higher power and has better performance. Moreover, it was proved that the addition of more than one intermediate electrode layer increases the pressure drop dramatically which fades the advantage of increasing power output.

THEORETICAL INVESTIGATION OF EGYPT CLIMATE CONDITIONS ON THE PERFORMANCE OF PEM FUEL CELL

This paper describes a theoretical model to investigate effects of different air parameters on the performance of PEM fuel cell. Parameters taken into considerations in the study are air mass flow rate, pressure, temperature and relative humidity. This study is applied on a fuel cell with a across section area of 218 cm2 stacked in a series of combined 110 cells. The observed effects on the fuel cell performance are presented as system power, efficiency and polarization curves. The stacked cells may produce an overall power output more than 20 kW, this model studies the effects in two cases the first with constant relative humidity effect, and the second with different relative humidity effect. This model monitor the effects on PEMFC performance by using the V-I polarization curves. It showed that with the increase of air pressure, temperature and relative humidity the power increases nearly up to 8 %, in the other hand with the increase of air flow rate the performance is decreasing.

(Invited) The FC-PAD Consortium: Advancing Fuel Cell Performance and Durability

The FC-PAD (Fuel Cell – Performance and Durability) consortium coordinates national laboratory activities related to fuel cell performance and durability, provides technical expertise, and integrate activities with industrial developers. The national laboratory core teams have the responsibility to carry out foundational research and capabilities development, and provide support for the individual projects’ research efforts. This consortium incorporates National-Laboratory investigators related to durability, transport, and performance, and combines them into one highly coordinated effort. The consortium formalized already existing and effective collaborations amongst the National Laboratories that established leadership in PEMFC performance and durability research and development. The consortium coordinates work under in different Thrust Areas including component thrust areas and cross-cutting thrust areas. The six different thrust areas are: Component Thrust Areas: Electrocatalysts and SupportsElectrode LayersIonomers, Gas Diffusion Layers, Bipolar Plates, Interfaces Cross-cutting Thrust Areas: Modeling and ValidationOperando Evaluation: Benchmarking, ASTs, and ContaminantsComponent Characterization and Diagnostics The structure of FC-PAD utilizes multiple cross-cutting thrust areas from theoretical modeling to characterization including benchmarking new materials that are provided to FC-PAD. The FC-PAD structure brings together world-class scientists into one integrated consortium, yet it provides a flexible structure to strategically use the widely varying expertise. The FC-PAD consortium is examining 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. Consortium members include Argonne National Laboratory, Lawrence Berkeley National Laboratory, Los Alamos National Laboratory, the National Renewable Energy Laboratory and Oak Ridge National Laboratory. Acknowledgments This work was funded through the DOE FC-PAD Consortium with thanks to DOE EERE FCTO, Fuel Cell Team Leader: Dimitrios Papageoropoulos

(Invited) Build Effective Cathode Nanocatalysts to Improve Performance of Proton Exchange Membrane Fuel Cells

Fuel cells (FCs) are green energy devices capable of conversion of chemical to electrical energy, releasing heat and water. One design of FCs are proton exchange membrane (PEM), which consist of a solid nafion membrane as the electrolyte, utilizing H2or bio-fuel as a fuel supply; and noble metal as the catalysts. PEMFCs provide direct electricity for stationary and portable applications with high energy conversion efficiency, low pollutant emissions, simple design and operation, and flexible fuel supply. However, current PEMFC designs encounter problems in materials compatibility, manufacturing cost, and performance degradation. Numerous attempts have been made to improve the performance and durability of PEMFCs. Commonly-used catalysts are carbon supported Pt and its alloy, but are plagued by high expense and low resistance to carbonaceous species or use of Pt alloy (which has low resistance to corrosion). Thus, use of non-Pt metal catalysts is desired; however, their durability and carbonaceous species tolerance is unknown requiring further investigation. The novelty of this approach is application of ACNTs which allows improved performance due to rapid gas diffusion and chemisorption of the gas reactants. ACNT hydrophobicity also provides a new tool to prevent cathode flooding, resulting in long-term device stability. The aligned carbon nanotubes are synthesized using chemical vapor deposition (CVD) with a xylene-ferrocene solution as the precursor. Xylene is the carbon source, while ferrocene provides the iron metal nanoparticles, which function as the seeds for the nanotubes growth. Three 5 cm2quartz substrates are placed inside an 1 inch diameter quartz reaction tube. The tube is placed in a two stage furnace and tightly sealed to air. The first stage of the furnace is at a temperature of 225 ºC, which is enough to vaporize the solution. The second stage is held at 725 ºC and is used to carbonize the vaporized solution, depositing the iron nanoparticles on the quartz substrates, and allowing the carbon nanotubes to grow around the iron seeds. The solution with the chemicals is injected into the reaction tube, on the low temperature stage, using argon and hydrogen as the carrier gases, at flow rates of 100 and 50 mL/min respectively. The chemicals injection rate is 0.225 and 0.250 mL/min. The anodic and cathodic materials were then place on two sides of the Na-form Nafion membrane. Hot pressing technique was then used to fabricate the membrane electrode assembly (MEA) under temperature of 210 ºC and pressure of 600 pound-forces per square inch gauge (pisg) for 5 to 10 mins. The surface morphology, cross-sectional images and the thickness of the aligned carbon nanotubes were determined using a field emission scanning electron microscope (JSM7600) equipped with X-ray energy dispersive spectroscope (Chemistry Department, Texas A&M University-Kingsville). Raman/Fourier transform infrared spectroscopy (FTIR, Hariba Jobin-Yvon LabRam IR system) confocal microscope (Materials Characterization Facility, Texas A&M University) was employed to obtain highly specific fingerprints for precise chemical and molecular characterization and identification. The single cell is attached to an Electrochem Inc. test stand to record the I-V polarization curves, which gives a measurement of the MEA’s performance. The PEMFCs device is initially conditioned for about 2 hours until it reaches the desired temperature and humidity conditions, using argon gas in the anode and oxygen gas in the cathode, at a constant voltage of 0.4 V, according to the US Fuel Cell Council test protocol. After a constant current is reached, the polarization curves are measured by potentiostatically cycling the voltage between 0.2 and 1 V. The Pt-functionalized aligned carbon nanotubes cathodic catalyst was obtained by the chemical vapor deposition, wet chemistry impregnation, and followed by heat-treatment at 300 °C. The catalyst was characterized through scanning electron microscopy and Raman spectroscopy to determine the unique structure and molecular interaction between C and Pt atoms. The results show that the dense nanotubes layers have been directly grown on cathode materials with highly aligned structure, which favors the gas diffusion. The diameter of the tubes is observed approximately 20-50 nm and its length varies from 10-30 µm according to the different growth time period. The Pt nanoparticles are uniformly distributed on the surface of tubes, whose size is ranged from 1 nm to 8 nm. The single PEMFC displayed the maximum power density >850 mV/cm2 and >320 mV/cm2 with the O2 and air oxidant introduced into cathode, respectively. Figure 1

(Invited) From Fundamentals to PEMFC Systems

Search for active and durable, but cost effective catalysts that can be employed in fuel cells remains one of the main challenges for broad application of this technology. This presentation intends to provide an overview of progress that has been made in design of Pt-based electrocatalysts that are currently used in low temperature fuel cells. The main focus will be placed on necessity to resolve current limitations at molecular scale, which can ultimately lead towards design of electrocatalysts with advanced properties. A major challenge lies in the insufficient activity and durability of materials that are currently employed as cathode catalysts for electrochemical reduction of oxygen. These limitations inevitably lead to a lower operating efficiency of the devices, which highlights the need for development of more active and durable oxygen reduction reaction catalysts. Consequently, the majority of research efforts are placed on the catalyst design and synthesis aiming to improve their efficiency. It has been found that properties such as surface structure, surface and subsurface composition associated with electronic properties have distinguished roles in determining functional properties of electrocatalysts. In addition to the catalyst materials it is of paramount importance to emphasize the role of liquid phase which is influencing on the overall properties of an electrified interface. Molecular species from employed electrolyte and the nature of their interaction with catalyst surfaces can be used in tuning the PEMFC performance.

Evaluation of performance enhancement by condensing the anode moisture in a proton exchange membrane fuel cell stack

Water management is an important issue for proton exchange membrane fuel cells. Back-diffusion of water from cathode to anode often occurs due to the differences in concentration and pressure during operation of fuel cell, resulting in the flooding and severe carbon corrosion in the cathode. Herein, we report a novel method of anode moisture condensing (AMC) in which a condenser is set at the outlet of the anode to cool down the anode moisture. With the help of AMC, liquid water is condensed from the moisture due to the variation of the saturated pressure of water vapor, which can accelerate the evaporating of the liquid water inside the anode and mitigate the probability of water flooding. A ten-cell stack with a condenser at the outlet of the anode is fabricated to systematically investigate the effects of the stack temperature and flow rate on the stack performance. The result shows that the PEMFC performance can be greatly improved at high current density and high operation temperature under the condition of AMC. The stack exhibits very similar performance before and after application of AMC below 500mAcm−2, whereas the output power increases from 405W to 436W at 600mAcm−2 at 65°C. With further increase in operation temperature to 80°C, the average voltage increases from 0.598V to 0.641V even at 500mAcm−2. Moreover, the application of AMC can speed up the water evaporation, leading to the dehydration of the membrane and thus poor performance of PEMFC at low current density.

Impact of the patterned membrane morphology on PEMFC performances of ultra-low platinum loaded MEAs

Two types of patterns were achieved on the surface of NR211 membranes: holes and knolls. These patterns were produced on only one side of the membrane at the cathode. For equivalent specific surfaces, these two membrane patterning morphologies were tested in fuel cell conditions with very low platinum loading (30 μg/cm2). Catalyst loading was achieved by magnetron plasma deposition either on the microporous carbon electrode or directly on the patterned membrane. The fuel cell performances in dry conditions were found to be highly dependent on the morphology of the membrane surface. Results showed that the knoll morphology gave better fuel cell performances than the hole morphology. For the knoll morphology the current density increased by a factor of 1.78 at 0.7 V versus a pristine membrane, whereas the hole morphology appeared in some cases to deteriorate the fuel cell performances, despite an increase in the specific surface by a factor of 1.87 versus a pristine membrane. The concepts of top level (extreme surface of the patterned membranes) and bottom level (the pattern base) were introduced to highlight the phenomena of micro water management on the areas of the micro-patterns which act on the local conductivity of the protonic membranes. These results suggest that it is necessary to choose the morphology of the patterns very carefully before simply increasing the specific surface of protonic membranes.

Charting the relationship between phase type-surface area-interactions between the constituent atoms and oxygen reduction activity of Pd–Cu nanocatalysts inside fuel cells by in operando high-energy X-ray diffraction

HE-XRD elucidates the link between structural changes of catalysts inside PEMFCs and the performance of PEMFCs.

Hierarchically ordered arrays with platinum coated PANI nanowires for highly efficient fuel cell electrodes

A hierarchically ordered electrode architecture with crystallographically preferential growth of Pt nanowhisker arrays on PANI nanowire arrays demonstrates outstanding ORR and PEMFC performance.

A framework for the probabilistic analysis of PEMFC performance based on multi-physical modelling, stochastic method, and design of numerical experiments

In this article, a probabilistic approach is applied to evaluate the impact of the GDL porosity uncertainty on the electrical performances of a PEMFC. The study is based on the use of a dynamical, symbolic, and acausal knowledge model. Some statistical distributions are introduced on the input model parameter (porosity) and the statistical distributions induced on the output parameters (cell voltage, resistance) are analyzed. The difference observed between the shapes of the input and output distributions (respectively some Gaussian and inverse Gamma distributions with a threshold phenomenon) is the result of strong nonlinearities linked with the integration of multiphase flow phenomena in the modelling (e.g. diffusion limit of the humidified air in the GDL). The study is also conducted for different conditions of temperature and pressure through a design of numerical experiments. One of the results obtained is that the variation coefficient related to the GDL porosity has, compared to the other parameters with their intervals of variation considered, little effect on the average output distributions. However, the dispersion introduced on the porosity impacts their shapes (e.g. significant effect on the standard deviation).

Design of active Pt on TiO2 based nanofibrous cathode for superior PEMFC performance and durability at high temperature

Oxygen reduction reaction (ORR) activity and stability of the cathode catalyst are important issues for practical applications, which should be even considered for the materials in high temperature polymer electrolyte membrane fuel cells (HT-PEMFCs). To improve these properties, modification of the catalyst electronic structure and finding durable supports can be a good approach. In this study, we synthesized a noble nanofibrous composite electrode which consist of carbon nanotube (CNT)-winded Pt/TiO2 nanofiber (CNT-Pt/TiO2). Our approach takes advantages of the electrochemical conductivity of CNF as well as better stability from the corrosion resistivity of TiO2 and strong metal-support interaction (SMSI) between the Pt nanoparticles and TiO2 nanofibers for less Pt dissolution. We also found that the Pt electronic state can be changed by an interaction with neighbouring CNT and TiO2, resulting a decrease of Pt d-band vacancy for enhanced catalytic activity. Furthermore, nanofibrous structure with a unique 3D pore structure provides higher surface area for additional improvements of the mass transfer. These results reveal that the CNT-Pt/TiO2 nanofiber based electrode shows enhanced performance with the maximum power density of 567mWcm−2 compare to commercial Pt/C (461mWcm−2) with a significant durability at harsh conditions of 120°C and RH 40%.

Experimental Investigation and Numerical Determination of Custom Gas Diffusion Layers on PEMFC Performance

The objective of this work is to investigate the design of gas diffusion layer (GDL) for fuel cells that possess specific transport properties. Various types of custom GDLs that have a microporous bilayer (MP) were studied. These GDLs have two different MP layers and a carbon substrate. GDLs with different bilayer MPs designs were tested and the fuel cells performance was investigated. The performance of the fuel cell was measured in-situ with different GDLs at several operating conditions. The pore morphology of the GDLs as well as pore size distribution of the GDLs including the MPs was determined. The data on the pore size distribution including the relationship between tortuosity and porosity was used as model parameters. The experimental data and model predictions confirm that introducing the bilayer MPs on the substrate could improve the transport resistant and increase the performance of PEMFC. The effect of carbon particle size and overall diffusivity also modified the value of MacMullin number and overall thickness of GDL causing the differences in performance and local distributions.

Numerical modelling of in-line and staggered blockages in parallel flowfield channels of PEM fuel cells

A blockage in the flowfield channel of a PEM fuel cell can enhance mass transfer of reactant gas from the channel into the catalyst layer and improve cell performance. Here, we wish to identify an optimal lay-out for the placement of blockages in the cathode side flow channels of PEM fuel cells. For this purpose, the effects of in-line and staggered blockage configurations within a parallel flowfield are numerically investigated and their fuel cell performance results are compared with those of a baseline parallel flowfield without blockages. A 3D, multiphase, non-isothermal numerical simulation of a single 9-layer counter-flow PEM fuel cell is performed. Results show that the staggered configuration enhances maximum net power by up to 11% over the baseline case, and by 7% when compared to the in-line case. The presence of over-rib-convection in the staggered configuration reduces the pressure drop by 70% when compared to the in-line case which only experiences over-block-convection. Finally, the effect of gas diffusion layer properties is investigated on PEMFC performance enhancement. Results of this numerical study are consistent with the results from our previous experimental study (Heidary et al. (2016)).

Experimental study of variable operating parameters effects on overall PEMFC performance and spatial performance distribution

Back pressure, relative humidity and air stoichiometry, these operational parameters have a crucial influence on the performance of PEMFC. Overall performance, stability and homogeneity of spatial performance distribution under variable operating parameters are in this paper researched. Polarization curves and Electrochemical Impedance Spectroscopy (EIS) are utilized to show the overall performance and resistance losses of PEMFC at variable operating parameters, while the segmented cell technique can provide real-time spatial current density distribution to reflect the uniformity and stability of the performance. Higher back pressure is an effective approach to improve the overall performance of the fuel cell whereas the uniformity keeps constant and the local stability decreases. The back pressure should be limited to some extent (0.3 bar in this paper). With the relative humidity increasing to a value (70% in this paper), overall performance and current distribution homogeneity of PEMFC enhance, but the stability sustains. Under higher air stoichiometry the fuel cell shows better overall and local performance, current distribution homogeneity and local stability. It's found that in all cases the performance near the outlet parts are worst due to poor water management and low gas concentration which could not be effectively enhanced by optimization of these operating conditions.